Complement of a set
The complement of a subset $$A$$ of $$E$$ consists of all the elements in $$E$$ that are not in $$A$$. It is written as $$A'$$ or $$A^c$$.
The complement of the set of the odd numbers is the set of even numbers (with the set of integers as the reference set).
The complement of the set of the vowels is the set of the consonants (with the set of English letters as the reference set).
The reference set $$E$$ is often omitted when it is clear which set we are referring to.
The complement of the complement of a set is the original set $$$ (A')' = A.$$$