Vocabulary - Complement

The complement of set $A$ (written $A^c$ or $\bar{A}$) is the set of all elements not in $A$, relative to some universal set $U$.

For example, if $U = \{1, 2, 3, 4, 5\}$ and $A = \{1, 2\}$, then $A^c = \{3, 4, 5\}$. You include everything in $U$ except what is in $A$.

Complement depends on the universal set. Without specifying $U$, the complement is undefined.