Definition of

Complement (set)

All elements (from a Universal set) NOT in our set.

Example: With a Universal set of all faces of a dice {1,2,3,4,5,6}
Then the complement of {5,6} is {1,2,3,4}.

Symbol is a little dash in the top-right corner.
Example: A' = {1,2,3,4}

Or a little "C" in the top-right corner.
Example: A^C = {1,2,3,4}

Together the set and its complement make the Universal set.

Another Example: In the set below the Universal set is {alex, blair, casey, drew, erin, francis, glen, hunter, ira, jade} and the complement of set "S" is:
S^C = {blair, erin, francis, glen, ira, jade}

See: Complement (probability)