Term frequency (${\mathrm{tf}}_{t,d}$)

The raw term frequency ${\mathrm{tf}}_{t,d}$ counts occurrences of term $t$ in one document $d$, after the chosen indexing preprocessing. It records how often the term appears there. Document frequency instead counts how many documents contain the term.^[ref]

For indexed terms $w_1,\dots ,w_L$, the indicator function givesIn red red boat, the raw term frequency of red is two and that of boat is one. The proportions $2/3$ and $1/3$ are normalized counts; they are not the raw counts in this convention.

Reordering the document to boat red red preserves both term frequencies but changes which phrase queries match. Counts therefore compress away ordering information. A positional index can retain the occurrence positions as well as the counts.^[ref]

A retrieval score can use term frequency as evidence that a document discusses a query term, but repetition alone does not establish relevance. Nor must doubling a raw count double a score: a scoring rule can deliberately reduce the effect of repeated occurrences. Whenever a transformed or normalized quantity is called term frequency, its formula should be specified.