a kind of associative container that supports equal keys (possibly contains multiple copies of the same key value) and provides for fast retrieval of the keys themselves

an unordered collection of values

a set for which repeated elements are considered

a set with (possibly) repeated elements

In mathematics, a multiset (sometimes also called a bag) differs from a set in that each member has a multiplicity, which is a natural number indicating (loosely speaking) how many times it is a member, or perhaps how many memberships it has in the multiset. For example, in the multiset { a, a, b, b, b, c }, the multiplicities of the members a, b, and c are respectively 2, 3, and 1.