In set theory, the complement of a set A, often denoted by A∁, is the set of elements not in A.When all elements in the universe, i.e. all elements under consideration, are considered to be members of a given set U, the absolute complement of A is the set of elements in U that are not in A. The relative complement of A with respect to a set B, also termed the set difference of B and A, written B ∖ A, {\displaystyle B\setminus A,} is the set of elements in B that are not in A. Wikipedia