A Combination is an unordered subset of a set.

For example, the 10 possible combinations of 3 letters from the set of vowels { A, E, I, O, U } are:

AEI, AEO, AEU, AIO, AIU, AOU, EIO, EIU, EOU and IOU.

 The use of the word "Combination" with reference to combination locks is incorrect as the code is actually a permutation - the order matters.
Order does not matter, so the choices "AEI" and "EAI" (for example) are considered to be the same.

The calculation of the number of combinations makes use of the factorial notation, binomial coefficients and Pascal's triangle.