[A Vector Space Model for Automatic Indexing](http://www.cs.uiuc.edu/class/fa05/cs511/Spring05/other_papers/p613-salton.pdf) (1975) by G. Salton, A. Wong, and C. S. Yang.

The first two paragraphs (after the abstract) describe how to compute a similarity coefficient between two documents. The documents are associated with one or more index terms; a weight for each term may be computed according to its importance in the given document; the similarity of the documents may be computed by comparing the resulting weights.

Instead of "entities," the paper refers to "index terms."