BM25 is a probabilistic weighting scheme used by Xapian that combines previous models (BM11 and BM15) with a scaling factor to improve term weighting in information retrieval. Recent TREC tests have shown it to be the best known probabilistic weighting scheme, with default parameters of k1=1, k2=0, k3=1, and b=0.5 that can be tuned for specific collections and query types.