Set operation (Absolute complement)

Sometimes we may consider all sets as being subsets of some given universal set, $U$.

Given a universal set $U$ and $A\subseteq U$, the complement of $A$, $\bar{A}$, is the set $$ \begin{aligned} \bar A &= U - A \\ &= \{ x \in U | x \notin A \} \end{aligned} $$

Venn diagram of $\bar{A}$