Associative containers are ordered containers. These containers include member functions that allow key insertion, retrieval, and manipulation. The Standard Library has the map, multimap, set, and multiset associative containers. map and multimap associate values with the keys and allow for fast retrieval of the value, based upon fast retrieval of the key. set and multiset store only keys, allowing fast retrieval of the key itself.
For more information about associative containers, see the Containers section of this Reference Guide, or see the entry for map, multimap, set, or multiset.
ISO/IEC 14882:1998 -- International Standard for Information Systems -- Programming Language C++, Section 23.3