In set theory, the complement of a subset of a given set is the "remainder" of the larger set.
Formally, if A is a subset of X then the (relative) complement of A in X is
In some version of set theory it is common to postulate a "universal set" and restrict attention only to sets which are contained in this universe. We may then define the (absolute) complement
The relation of complementation to the other set-theoretic functions is given by De Morgan's laws: