Vocabulary - Union

The union of sets $A$ and $B$ is the set of all elements in either $A$ or $B$ or both. You write $A \cup B$.

For example, if $A = \{1, 2\}$ and $B = \{2, 3\}$, then $A \cup B = \{1, 2, 3\}$. Notice that $2$ appears in both sets, but the union lists it once because sets have no duplicates.

Union merges sets together. In code, this is like adding all items from two lists into one set, removing duplicates automatically.