The Limit. The set $A\times B$ defined by
\begin{align*} A\times B & \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\prod _{z\in \webleft\{ A,B\webright\} }z\\ & \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\webleft\{ f\in \mathsf{Sets}\webleft (\webleft\{ 0,1\webright\} ,A\cup B\webright )\ \middle |\ \text{we have $f\webleft (0\webright )\in A$ and $f\webleft (1\webright )\in B$}\webright\} \\ & \cong \webleft\{ \webleft\{ \webleft\{ a\webright\} ,\webleft\{ a,b\webright\} \webright\} \in \mathcal{P}\webleft (\mathcal{P}\webleft (A\cup B\webright )\webright )\ \middle |\ \text{we have $a\in A$ and $b\in B$}\webright\} \\ & \cong \webleft\{ \begin{aligned} & \text{ordered pairs $\webleft (a,b\webright )$ with}\\ & \text{$a\in A$ and $b\in B$}\end{aligned} \webright\} .\end{align*}