The complement of B relative to A is standardly written "A\\B". Formally:
For example, {1,2,3}\\{2,3,4} is {1} and {2,3,4}\\{1,2,3} is {4}.
If a universal set U is defined, then absolute complement (or simply complement) of a subset B of U is just U \\ B and is denoted by B'.
For example, if the universal set is the set of natural numbers, then the complement of the set of odd numbers is the set of even numbers.
If both A and B are subsets of U then A\\B=A∩B' (the intersection of A and the complement of B).