What is semantic search in IR?

Semantic search in information retrieval (IR) refers to techniques that aim to understand the intent and contextual meaning behind a user’s query rather than relying solely on keyword matching. Unlike traditional keyword-based search, which matches documents based on exact terms, semantic search interprets the relationships between words, concepts, and user intent to return more relevant results. This approach uses natural language processing (NLP) and machine learning models to analyze the semantics—or meaning—of both the query and the content being searched. For example, a query like “how to fix a leaking pipe” might retrieve results about plumbing repairs, even if the exact phrase “leaking pipe” isn’t present in the documents.

At a technical level, semantic search often involves representing text as vectors in a high-dimensional space. Models like BERT, Word2Vec, or sentence transformers convert words, phrases, or entire documents into numerical embeddings that capture semantic similarities. For instance, the word “automobile” and “car” would be close in this vector space, even if they don’t share letters. When a user submits a query, the system computes its vector and compares it to the vectors of indexed documents using similarity metrics like cosine similarity. This allows the retrieval of documents that are conceptually related but might lack exact keyword overlaps. Techniques like latent semantic indexing (LSI) or transformer-based architectures (e.g., SBERT) are commonly used to build these representations. Developers might implement this using libraries like Hugging Face Transformers or FAISS for efficient similarity search.

A practical example of semantic search is in e-commerce. If a user searches for “wireless headphones under $100,” a semantic system could identify products labeled as “Bluetooth earbuds” or “cord-free earphones” priced at $99, even if those exact terms aren’t in the query. Another use case is enterprise search, where employees might look for “Q4 sales reports” and receive results containing “Fourth-quarter revenue analysis” documents. Tools like Elasticsearch with semantic plugins or custom solutions using OpenAI’s embeddings enable developers to integrate semantic search into applications. By focusing on meaning rather than syntax, semantic search improves relevance, handles synonyms and ambiguities, and adapts to natural language variations—making it particularly useful for modern applications requiring nuanced understanding.