combination

A combination is a subset of a given set where the order of elements is ignored (as distinct from a permutation).

For example there are 6 combinations of size 2 from the set {1, 2, 3, 4}:
{1, 2}, {1, 3}, {1, 4}, {2, 3}, {2, 4}, {3, 4}.

The number of combinations of size $k$ from a set of size $n$ is the binomial coefficient also written $_{n}C_{k}$.