What is precision at k in retrieval evaluation?

Precision at k is the proportion of retrieved top-k documents that are actually relevant to the query. It measures whether the retriever ranks relevant documents near the top, which matters when only the top few are passed to the generator. Precision at 3 is particularly important in RAG because LLM context windows often force limiting to three or five chunks. Precision is less critical than recall because irrelevant documents can be filtered by a reranker, but relevant documents missed in recall cannot be recovered.