What is hybrid search and how does it combine dense and sparse retrieval?

Hybrid search runs both a dense vector query and a sparse lexical query like BM25 in parallel, then fuses the two result lists into a single ranking. Dense retrieval catches semantic matches where users phrase queries differently from documents, while sparse retrieval catches exact keyword matches on names, identifiers, and rare terms. Fusion uses either weighted score combination or Reciprocal Rank Fusion, which combines ranks rather than raw scores. Hybrid search typically lifts recall 5 to 15 percent over either method alone.