Multimap

A map simpliciter is a data structure which defines a unique correlation of keys to values. A multimap is a generalization in which more than one value may be associated with a given key. For example, any given finite bit stream has a single, unique MD5 hash associated with it. Thus, the MD5 function defines a map from bit stream keys to hash values. In contrast, the index of a book may report any number of references for a given index term, and thus is an example of a multimap from index terms to any number of reference locations.