Reciprocal Rank Fusion (RRF) merges ranked lists from different retrieval systems — such as BM25 and dense retrieval — by assigning each result a score of 1/(k + rank) and summing across lists. The constant k (typically 60) dampens the influence of top ranks. RRF requires no training, no score normalization, and consistently outperforms individual rankers, making it the default fusion method in most hybrid search implementations.