n. 1. also called unordered arrangement. A selection of a subset of objects from a set without regard to order. If repetitions are not permitted, the number of distinct combinations selecting r objects out of n is
(also written _nC_r or ⁿC_r). For example, the distinct combinations of two objects selected from {a, b, c, d} are ab, ac, ad, bc, bd, cd, since db counts as the same selection as bd; there are 15!/(12! × 3!) = 455 ways of selecting 12 jurors from a panel of of 15. Compare arrangement, permutation. See also binomial theorem, Pascal's triangle. 2. in a structure such as a vector space, any finite sum of appropriate multiples of given elements, such as a linear combination, affine combination, or convex combination.