What is TF*IDF (Term Frequency-inverse Document Frequency)?

Tf*Idf (Term Frequency-inverse Document Frequency) is TF*IDF (term frequency-inverse document frequency) is a measure of the relative centrality of a certain keyword to a particular web page based on its frequency in comparison to a collection of similar pages. A numerical statistic that is used by the search engine to determine the topical relevance of the web page. It checks the number of times a keyword (and any LSI keyword) has been used in the content of one document and cross-references it with all the other documents in the group.

From Simba’s SEO Dictionary