BM25

Gold definitionUpdated Apr 2, 2026

Definition

BM25 is a ranking function used in information retrieval to estimate the relevance of documents to a given search query. It's a sparse retrieval method that calculates a score for each document based on term frequency, inverse document frequency, and document length normalization.

At a glance

Executive summary

BM25 is a fundamental search algorithm that helps find the most relevant documents for a given query. It's often used in AI systems, especially those that combine large language models with external knowledge, to ensure the AI has accurate information and avoids making things up.

TL;DR

BM25 is a classic search algorithm that ranks how relevant documents are to a query, often used in AI systems to help them find facts.

Key points

Ranks documents based on term frequency, inverse document frequency, and document length normalization.
Effectively retrieves relevant documents for a query, mitigating vocabulary mismatch.
Used by information retrieval researchers, ML engineers, and developers of RAG systems and search engines.
As a sparse retriever, it complements dense (embedding-based) retrievers, often combined for better performance.
Increasingly integrated into hybrid retrieval strategies for RAG systems to enhance LLM grounding and reduce hallucinations.

Use cases

Enterprise Search: Powering internal search engines for large organizations to find relevant documents or knowledge base articles.
E-commerce Product Search: Ranking products based on user queries, considering product descriptions, features, and reviews.
Legal Document Discovery: Identifying highly relevant legal precedents or case documents for specific legal queries.
Medical Information Retrieval: Helping clinicians and researchers find relevant medical literature or patient records for diagnostic support.
Retrieval-Augmented Generation (RAG): Providing initial document candidates to LLMs for question answering, summarization, or content generation tasks.

Also known as

Okapi BM25, BM25F, BM25+