Elasticsearch

Okapi BM25 is a classic, probabilistic ranking function used in information retrieval to score document relevance based on exact keyword matching, term frequency, and inverse document frequency.

A unified retrieval approach that combines sparse lexical matching and dense vector search, typically merged via scoring algorithms like Reciprocal Rank Fusion (RRF).

Traditional keyword-based text search that matches exact words, substrings, or tokenized stems using inverted index structures.