Definition:Combination

Definition
Let $S$ be a set containing $n$ elements.

An $r$-combination of $S$ is a subset of $S$ which has $r$ elements.

Also see

 * Cardinality of Set of Subsets: the number of $r$-combinations of $S$ is $\dfrac {n!} {r! \left({n - r}\right)!} = \dbinom n r$