The union[1] of $A$ and $B$ is the set $A\cup B$ defined by
\[ A\cup B \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\bigcup _{z\in \webleft\{ A,B\webright\} }z. \]
Footnotes
[1] Further Terminology: Also called the binary union of $A$ and $B$, for emphasis.
