1. <mathematics> A set containing a certain number of objects selected from another set.
The number of combinations of r objects chosen from a set of n is
n C r = n! / ((n-r)! r!)
where "n C r" is normally with n and r as subscripts or as n above r in parentheses.
See also permutation.
2. <reduction> In the theory of combinators, a combination denotes an expression in which function application is the only operation.