For questions about or related to multisets, a notion similar to sets with the difference that elements can be repeated.

A *multiset*, also called a bag, is a generalization of a set in which elements can appear more than once. Formally, a multiset can be defined as a pair $(A, m)$ where $A$ is a set and $$m : A \to \{1, 2, 3, ...\}$$

For each $a \in A$, the number $m(a)$ is called the multiplicity of $a$, and counts how many times $a$ appears in the multiset.

Reference: Multiset.