IDF反映了在文档集合中一个单词对一个文档的重要性，经常在文本数据挖据与信息提取中用来作为权重因子。在一份给定的文件里，词频(termfrequency-TF)指的是某一个给定的词语在该文件中出现的频率。逆向文件频率（inversedocument frequency，IDF）是一个词语普遍重要性的度量。-IDF reflects the importance of a word in a document collection for a document, often in the text, according to data mining and information extraction is used as a weighting factor. In a given file, word frequency (termfrequency-TF) refers to the frequency of a given certain word appears in the document. Inverse document frequency (inversedocument frequency, IDF) is a measure of the importance of a common word.