BM25.

also: TF-IDF · IDF · inverse document frequency · term frequency · relevance score · relevance scoring

The standard relevance score: rewards terms frequent in a document (with diminishing returns) and rare across the corpus, normalising for length.

the two-minute refresher

Term frequency: more occurrences = more relevant, but 10 isn't 10× better than 1 (saturation, k1).
IDF: rare words matter more than common ones.
Length normalisation (b): long documents don't win just by containing more words.
The default scorer in Lucene and Elasticsearch.

also: TF-IDF · IDF · inverse document frequency · term frequency · relevance score · relevance scoring

The standard relevance score: rewards terms frequent in a document (with diminishing returns) and rare across the corpus, normalising for length.

Term frequency: more occurrences = more relevant, but 10 isn't 10× better than 1 (saturation, k1).
IDF: rare words matter more than common ones.
Length normalisation (b): long documents don't win just by containing more words.
The default scorer in Lucene and Elasticsearch.