A Permutation is an ordered subset of a set.

For example, if the subset of the alphabet are the letters A, C, and T then the six possible permutations of this subset are: ACT, ATC, CAT, CTA, TAC and TCA.

The use of the word Permutations (Perms) with respect to football pools is incorrect as they are really combinations.

The calculation of the number of permutations makes use of the factorial notation.