When we are working in an area or on a certain problem, we always have a frame of reference in which we are working called a universal set. In our geometry course, it will be the set of points that lie on a plane. In calculus we consider the set of real numbers, the set of real functions, the set of differentiable functions, and the set of continuous functions as universal sets.
The complement of a set A is defined to be the set of all elements of
the universal set which are not in A, and is symbolized by 
. Note
that 
is always the universal set, while 