The union of two sets A and B, denoted A∪B, is the set of things which are either in A or in B or both.

Formally stated, where C=A∪B
x∈C↔(x∈A∨x∈B)
That is, Iff x is in the union C, then either x is in A or B or possibly both.
Examples
- {1,2}∪{2,3}={1,2,3}
- {1,2}∪{8,9}={1,2,8,9}
- {0,2,4,6}∪{3,4,5,6}={0,2,3,4,5,6}
- R−∪R+∪{0}=R (In other words, the union of the negative reals, the positive reals and zero make up all of the real numbers.)