Chapter 2 - Set Theory - 2.3 Venn Diagrams and Set Operations - Exercise Set 2.3 - Page 83: 150

The union of two sets is a set that contains all elements that are either in one of or are in both of the sets.

The union of two sets is a set that contains all elements that are either in one of or are in both of the sets. For example: $A=\{0, 4\}$ $B=\{1, 2, 3\}$ $A\cup B=\{0, 1, 2, 3, 4\}$