What is the Union in math?

What is the Union in math: The union of two sets is a new set that contains all the elements from both of the original sets.

For example, if Set A = {1, 2, 3} and Set B = {3, 4, 5}, then the union of Set A and Set B is Set A ∪ B = {1, 2, 3, 4, 5}.