What is chunk overlap and why is it used?

Chunk overlap is the number of tokens or characters repeated between adjacent chunks, typically 10 to 20 percent of chunk size. It prevents losing information when a sentence or concept straddles a chunk boundary, which would otherwise split context across two chunks and hurt retrieval for queries about that exact topic. Overlap adds storage cost and creates near-duplicate results during retrieval, which reranking or deduplication can handle. A common default is 50 to 100 tokens overlap on a 512-token chunk.