## combination

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.