From wikipedia.org英文版，我主要将其改变成中文。
BM25（Best Match25）是在信息检索系统中根据提出的query对document进行评分的算法。It is based on the probabilistic retrieval framework developed in the 1970s and 1980s by Stephen E. Robertson, Karen Spärck Jones, and others.BM25算法首先由OKapi系统实现，所以又称为OKapi BM25。
BM25属于bag-of-words模型，bag-of-words模型只考虑document中词频，不考虑句子结构或者语法关系之类，把document当做装words的袋子，具体袋子里面可以是杂乱无章的。It is not a single function, but actually a whole family of scoring functions, with slightly different components and parameters. One of the most prominent instantiations of the function is as follows.
对于一个query , 包括关键字
, 一个文档的BM25得分:
- 其中IDF是上篇文章《TD-IDF》中的IDF，f是《TD-IDF》中的TF，|D|是文档D的长度，avgdl是语料库全部文档的平均长度。k1和b是参数。usually chosen, in absence of an advanced optimization, as
and
。