information science A search based on frequencies of the

search terms in the documents being searched. Weighted

search is often used by search engines. It produces a

numerical score for each possible document. A document's

score depends on the frequency of each search term in that

document compared with the overall frequency of that term in

the entire corpus of documents. A common approach is called

tf.idf which stands for term frequency * inverse document

frequency. Term frequency means "the more often a term occurs

in a document, the more important it is in describing that

document."

http://ciir.cs.umass.edu/cmpsci646/ir4/tsld034.htm Inverse

document frequency means the more documents a term appears in,

the less important the term is.