BM25
BM25 (Best Matching 25) is a widely used keyword-ranking algorithm that scores how relevant a document is to a query based on term frequency, adjusted for document length and how common each term is across the catalog.
Part of Search Relevance
It's a standard baseline for lexical (keyword) search relevance and predates modern vector-based methods. Many hybrid search systems still use BM25 for the keyword-matching half of their ranking, combined with a separate semantic score.