BM25 (Okapi)
The foundational keyword-frequency ranking formula still used as one half of modern AI retrieval systems.
Plain Language Definition
BM25 is the standard keyword search formula that ranks pages based on how often search terms appear, with diminishing returns for repetition and adjustments for document length. It has been the backbone of search engines for decades and is still widely used as one half of hybrid retrieval systems. BM25 is why including your target terminology in headings and body copy still matters in 2026.
Technical Definition
Non-linear term-frequency ranking function with IDF weighting that applies saturation curves to term frequency (preventing keyword stuffing gains) and document-length normalization — the foundational lexical retrieval baseline in most modern search systems.
Why This Matters for AI Search Visibility
BM25 remains one component of hybrid retrieval in virtually every production search system including AI answer engines. Content that uses clear, consistent terminology still benefits from lexical matching even as semantic retrieval gains importance — making keyword discipline still relevant.