Term Frequency. Term frequency. Two factors: A term that appears just once in a document is probably not as significant as a term that appears a number of times in the document.
NB: frequency = count in IR
Will turn out the base of the log is immaterial.
There is one idf value for each term t in a collection.
Each document is now represented by a real-valued vector of tf-idf weights ∈ R|V|
|V| is the number of terms