A multiset is a generalization of the notion of set to include collection of objects in which each element may occur more than once. Elements are not required to appear more than once, so that although not every multiset is a set, every set is a multiset.

The elements of a multiset are enclosed by braces, just as in the case of sets (e.g., $\{-2, 1, \pi, 1, 3.4, -2, -2\}$ and $\{0, \textrm{apple}, \sqrt{2}$ are multisets).