What Is Semantic Search & How To Implement [Python, BERT, Elasticsearch]

by | Oct 17, 2023 | Data Science, Natural Language Processing

What is semantic search?

Semantic search is an advanced information retrieval technique that aims to improve the accuracy and relevance of search results by understanding the context and meaning of the search query and the content being searched. Unlike traditional keyword-based search, which relies on matching specific words or phrases, semantic search considers the query’s intent, context, and semantics.

Semantic search is precious in applications where precision and relevance in search results are critical, such as information retrieval from large databases, e-commerce product searches, enterprise search, and improving the user experience in search engines and virtual assistants.

Unlike traditional keyword-based search, which relies on matching specific words or phrases, semantic search considers the query's intent, context, and semantics.

While traditional keyword-based search relies on matching specific words or phrases, semantic search considers the query’s intent, context, and semantics.

How does semantic search work in NLP?

Semantic search in the context of natural language processing (NLP) refers to applying NLP techniques to enhance the accuracy and relevance of search results by understanding the meaning and context of the search queries and the content being searched. Here’s how semantic search is related to NLP:

  • Natural Language Understanding: NLP analyses and understands the natural language used in search queries and the content within the search database. NLP techniques, such as part-of-speech tagging, named entity recognition, and syntactic parsing, help extract meaning from text.
  • Query Expansion: Semantic search in NLP often involves query expansion, where the system identifies synonyms, related concepts, and contextually relevant terms to broaden the scope of the search query. This ensures that the search results are not limited to exact keyword matches but include conceptually related content.
  • Entity Recognition: NLP-based semantic search systems can identify entities (e.g., people, places, organizations) in the text and use this information to improve search accuracy. For example, recognizing that “Apple” refers to the technology company, not the fruit.
  • Conceptual Matching: NLP-powered semantic search goes beyond exact keyword matching and considers the underlying concepts and relationships between words and phrases. It can identify content conceptually related to the query, even if the terms are absent.
  • Sentiment Analysis: In some cases, sentiment analysis using NLP techniques can be incorporated into semantic search to determine the sentiment or emotional tone of the content, which can be important for specific search applications.
  • Contextual Understanding: NLP helps understand the context in which words and phrases are used, allowing the search system to provide contextually relevant results.
  • Multilingual Search: NLP enables semantic search to work in multiple languages by understanding the linguistic nuances and language-specific context.
  • Machine Learning: Machine learning models can be used in NLP-based semantic search systems to continually improve the relevance of search results based on user interactions and feedback.

Overall, semantic search with NLP offers more sophisticated and context-aware search capabilities, making it valuable in various applications, including web search engines, enterprise search, e-commerce, chatbots, and virtual assistants, where understanding and meeting the user’s intent is crucial.

What is an example of semantic search?

Here’s an example of a semantic search to illustrate how it works:

Scenario: Imagine you are using a semantic search engine to find information about “alternative energy sources” for your research project. In a traditional keyword-based search, you might simply enter the query “alternative energy sources” and get a list of results based on the exact match of those keywords. However, with semantic search, the results are more contextually relevant and conceptually driven.

Semantic Search Query: You enter the query “What are the most environmentally friendly alternative energy sources for residential use?”

Semantic Search Process:

  1. Natural Language Understanding: The semantic search system uses NLP to analyze your query. It understands that you are looking for environmentally friendly alternative energy sources for residential use.
  2. Query Expansion: The system expands your query by considering synonyms and related terms. It might include concepts like “green energy,” “renewable energy,” and “home energy solutions.”
  3. Contextual Understanding: The system identifies the context of your query, which is residential use and environmental friendliness. It knows that you’re not interested in industrial-scale energy solutions but those suitable for homes.
  4. Conceptual Matching: The semantic search system matches concepts and related terms instead of exact keyword matching. It searches for content that discusses renewable energy sources that are eco-friendly and practical for residential applications.

Search Results:

You receive a list of search results that include articles, reports, and products related to residential alternative energy sources. The results include solar panels, wind turbines, geothermal heating, energy-efficient appliances, and detailed information on their environmental benefits and practicality for homes.

The results may also include user-generated content, such as forum discussions and reviews, where people have shared their experiences with various residential alternative energy sources.

This example demonstrates how semantic search goes beyond simple keyword matching and considers the meaning and context of your query to provide more relevant and valuable search results. Understanding your query’s intent and nuances helps you find the information you need more effectively.

What is a semantic engine?

A semantic engine is a software system or component designed to understand, analyze, and process the meaning and context of human language. It is often used for natural language understanding (NLU), natural language processing (NLP), and semantic search. Semantic engines use various techniques and technologies to extract and work with the semantics of text and speech, allowing them to perform a wide range of tasks, including:

  • Natural Language Understanding (NLU): Semantic engines can understand the meaning of text or speech, enabling them to extract entities, recognize intents, and comprehend context. They are commonly used in chatbots, virtual assistants, and customer support systems to interact with users human-likely.
  • Semantic Search: Semantic engines can perform advanced search operations beyond simple keyword matching. They understand the context and intent behind search queries, making search results more relevant and accurate.
  • Information Retrieval: Semantic engines are used in systems that retrieve information from large databases or document collections. They can identify relevant documents and extract meaningful content.
  • Sentiment Analysis: These engines can determine a text’s sentiment or emotional tone, such as identifying whether a customer review is positive or negative.
  • Document Summarization: Semantic engines can summarise lengthy documents concisely by identifying essential information and significant details.
  • Machine Translation: Machine translation systems use semantic analysis to understand the meaning of sentences in one language and generate semantically accurate translations in another.
  • Named Entity Recognition (NER): They can identify and classify entities such as names of people, places, organizations, and more within text.
  • Topic Modeling: Semantic engines can group documents or texts into topics or clusters based on semantic content, helping with content categorization and organization.
  • Question Answering: They are used in question-answering systems that can comprehend questions in natural language and provide accurate responses by extracting information from documents or knowledge bases.
  • Recommendation Systems: Semantic analysis can help recommend products, content, or services to users based on their preferences and past behaviour.

To build a semantic engine, developers typically use natural language processing (NLP) and machine learning techniques, which can involve training models on large datasets and pre-trained language models like BERT, GPT-3, or specific domain models. These engines can be customized and fine-tuned to enhance performance for specific applications, domains, or languages.

Semantic engines are crucial in improving human-computer interactions, search, and information processing, making them an integral part of many modern applications and services.

How to implement semantic search in Python

You can use a combination of natural language processing (NLP) libraries and techniques to implement semantic search in Python.

Side Note: In a real-world scenario, you would typically work with more extensive datasets and possibly pre-trained models for better results. This example serves as a basic introduction.

1. Install Required Libraries:

You’ll need Python libraries such as spaCy and scikit-learn to perform semantic searches. You can install them using pip:

pip install spacy pip install scikit-learn 

2. Preprocess Your Data:

For semantic search, you should have a collection of documents or texts you want to search through. In this example, let’s assume you have a list of documents.

documents = [
    "Solar panels are a renewable energy source and are good for the environment.",
    "Wind turbines harness wind energy and generate electricity.",
    "Geothermal heating uses heat from the Earth to warm buildings.",
    "Hydropower is a sustainable energy source, relying on water flow for electricity generation.",
    # Add more documents as needed

3. Tokenization and Vectorization:

You need to tokenize the text and convert it into numerical vectors. In this example, we’ll use spaCy for tokenization and scikit-learn’s TF-IDF vectorization.

import spacy
from sklearn.feature_extraction.text import TfidfVectorizer

nlp = spacy.load("en_core_web_sm")

# Tokenize and vectorize the documents
tfidf_vectorizer = TfidfVectorizer()
tfidf_matrix = tfidf_vectorizer.fit_transform(documents)

4. User Query Processing:

Now, process the user’s query using spaCy to tokenize and vectorize it.

user_query = "What are the benefits of wind energy for the environment?"

query_vector = tfidf_vectorizer.transform([user_query])

5. Semantic Search:

Calculate the similarity between the user query and the documents using a similarity measure like cosine similarity. The higher the cosine similarity, the more similar the documents are to the user’s query.

from sklearn.metrics.pairwise import cosine_similarity

# Calculate cosine similarity between the user query and all documents
cosine_similarities = cosine_similarity(query_vector, tfidf_matrix)

# Get the index of the most similar document
most_similar_document_index = cosine_similarities.argmax()

6. Retrieve Results:

Once you have the most similar document index, you can retrieve the relevant document from your collection.

most_similar_document = documents[most_similar_document_index]
print("Most similar document:", most_similar_document)

This is a basic example of implementing semantic search in Python using spaCy and scikit-learn. Our next example will use a more advanced pre-trained model, BERT, to improve semantic understanding and search accuracy.

How to implement semantic search with BERT

Implementing semantic search using BERT (Bidirectional Encoder Representations from Transformers) involves using a pre-trained BERT model to generate embeddings for your documents and user queries and then calculating their similarity. Here’s a step-by-step guide on how to perform semantic search using BERT in Python:

1. Install Required Libraries:

You will need Hugging Face’s Transformers library to work with BERT models. You can install it with pip:

pip install transformers 

2. Preprocess Your Data:

You should have a collection of documents as before. Ensure you have the Hugging Face BERT model of your choice downloaded and ready to use. You can choose from various pre-trained BERT models, such as “bert-base-uncased” or “bert-large-uncased.”

3. Tokenize and Encode Documents:

Tokenize and encode the documents using the BERT tokenizer and model.

from transformers import BertTokenizer, BertModel
import torch

tokenizer = BertTokenizer.from_pretrained("bert-base-uncased")
model = BertModel.from_pretrained("bert-base-uncased")

# Tokenize and encode the documents
document_embeddings = []
for document in documents:
    inputs = tokenizer(document, return_tensors="pt", padding=True, truncation=True)
    outputs = model(**inputs)
    document_embedding = outputs.last_hidden_state.mean(dim=1)  # Average over tokens
document_embeddings = torch.cat(document_embeddings)

4. Tokenize and Encode User Query:

Tokenize and encode the user query in the same way as the documents.

user_query = "What are the benefits of wind energy for the environment?"

user_query_inputs = tokenizer(user_query, return_tensors="pt", padding=True, truncation=True)
user_query_outputs = model(**user_query_inputs)
user_query_embedding = user_query_outputs.last_hidden_state.mean(dim=1)

5. Semantic Search:

Calculate the similarity between the user query and the document embeddings. One common similarity metric is cosine similarity.

from sklearn.metrics.pairwise import cosine_similarity

# Calculate cosine similarity between the user query and all documents
similarities = cosine_similarity(user_query_embedding, document_embeddings)

# Find the index of the most similar document
most_similar_document_index = similarities.argmax()

6. Retrieve Results:

Retrieve the most similar document from your collection.

most_similar_document = documents[most_similar_document_index]
print("Most similar document:", most_similar_document)

This example demonstrates how to perform a semantic search using the BERT model to generate embeddings for documents and user queries and then calculate the similarity to find the most relevant document. BERT’s contextual understanding can significantly enhance the quality of search results compared to traditional methods.

How to implement semantic search with Elasticsearch

Elasticsearch, a popular open-source search and analytics engine, can be used to implement semantic search by leveraging its text analysis capabilities and various features. Elasticsearch provides the foundation for building sophisticated search applications that can understand and deliver contextually relevant search results. Here’s a high-level overview of how to implement it with Elasticsearch:

1. Install and Set Up Elasticsearch:

First, you need to install Elasticsearch and set up an Elasticsearch cluster. You can download Elasticsearch from the official website and follow your specific operating system’s installation and configuration instructions.

2. Index Your Data:

Elasticsearch works by indexing and searching documents. You’ll need to index the documents you want to perform a semantic search. To do this, you’ll define an Elasticsearch index and use Elasticsearch’s REST API or a client library to add your documents to the index.

For example, if you have a collection of articles, each can be a document in your Elasticsearch index. You’ll need to specify how the content of the documents should be analyzed and tokenized during indexing. To enable semantic search, you may want to use custom analyzers or language-specific analyzers that consider synonyms and other language-specific nuances.

3. Use Full-Text Search:

Elasticsearch provides a powerful full-text search feature that allows you to perform keyword-based searches on your indexed data. You can use the match query or multi_match query to search for specific keywords in the documents.

  "query": {
    "match": {
      "content": "renewable energy sources"

4. Implement Semantic Search:

To implement semantic search, you can extend Elasticsearch’s capabilities by incorporating semantic search components such as word embeddings, synonyms, or ontologies. Here are a few ways to achieve this with Elasticsearch:

  • Synonyms: You can create synonym lists and use Elasticsearch’s synonym token filter to expand the query terms to include synonyms. This allows Elasticsearch to find documents containing similar, but not identical, terms.
  •  Word Embeddings: You can use pre-trained word embedding models (e.g., Word2Vec or FastText) to represent words in a vector space. These embeddings can be used to find documents with similar semantic content. Elasticsearch doesn’t provide native support for embeddings, but you can use custom scripts or external libraries to perform similarity searches based on embeddings.
  • Custom Analyzers: You can create custom analyzers in Elasticsearch that consider semantic information, such as synonyms and stemming. This can help improve search results by understanding the context of the query.
  • Graph Database Integration: You can integrate Elasticsearch with a graph database like Neo4j for more complex semantic search applications. The graph database can store relationships between concepts and entities, while Elasticsearch can handle text search and retrieval.
  • Query Expansion: You can expand the user’s query by identifying related terms or concepts and modifying the Elasticsearch query accordingly to retrieve more contextually relevant results.
  • Evaluation and Tuning: Continuously evaluate the quality of your results and fine-tune your Elasticsearch configuration, analyzers, and search queries to improve the relevance of results.

Implementing semantic search with Elasticsearch can be a complex and ongoing process. Still, it offers powerful capabilities for improving the relevance of search results by understanding the meaning and context of queries and documents.


Semantic search and semantic engines represent advanced approaches to understanding and processing natural language, making it possible to extract meaning and context from text and speech. These technologies have a wide range of applications. They are instrumental in improving the quality and relevance of search results and enabling more natural and intelligent interactions between humans and machines.

Semantic search goes beyond traditional keyword-based search by considering the intent, context, and meaning behind queries. It leverages natural language processing (NLP) and techniques like query expansion, synonym recognition, and conceptual matching to provide more accurate and contextually relevant search results.

Semantic engines, powered by NLP and machine learning, are at the heart of semantic search and enable various applications, including natural language understanding, sentiment analysis, information retrieval, and recommendation systems. These engines can be tailored to specific domains, languages, and use cases, making them versatile tools for enhancing user experiences and automating information-processing tasks.

As technology advances, semantic search and semantic engines will likely play an increasingly crucial role in various industries, from e-commerce and customer support to healthcare and content recommendation. Their ability to understand human language and context nuances allows for more intuitive and efficient interactions between humans and machines.

About the Author

Neri Van Otten

Neri Van Otten

Neri Van Otten is the founder of Spot Intelligence, a machine learning engineer with over 12 years of experience specialising in Natural Language Processing (NLP) and deep learning innovation. Dedicated to making your projects succeed.

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