 $idf_t = log_{10} \frac{N}{df_t}$ where N is the total number of documents in the collection.

Inverse Document Frequency(IDF)

The document frequency $$df_t$$ of a term $$t$$ is the number of documents in a collection that contain the term.

University of Michigan - Ann Arbor

Google

 A term refers to a word in a collection, but it may also include phrases.

Term(In IR)

A document is whatever unit of text the system indexes and retrieves (web pages, scientific papers, news articles, or even shorter passages like paragraphs.

Document(In IR)

An on-going but a helpful book resource about NLP
https://web.stanford.edu/~jurafsky/slp3/

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