Definition:Multiset

A multiset is an extension of the concept of a set.

While a set can contain only one occurrence of any given element, a multiset may contain multiple occurences of the same element.

Note that by this definition, a set is also classified as a multiset.

Example
While $$\left\{{a, b, c, d}\right\}$$ is a set, $$\left\{{a, b, c, c, d}\right\}$$ is a multiset.