TITLE: In set theory, what does it mean for a variable to have a bar symbol above it?
QUESTION [8 upvotes]: Please see the image below.

What does the bar symbol mean in this context?

REPLY [10 votes]: The complement of the set $B$, also commonly denoted as $B'$ or $B^c$. It is the set:
$$\bar{B} = B^c = B' = \{x \mid x \not \in B\}$$