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.

From Cardinality of Set of Subsets, we have that the number of $$r$$-combinations of $$S$$ is $$\frac {n!} {m! \left({n - m}\right)!} = \binom n m$$.