\begin{definition}[Definition:Commutative Square]
Let $\CC$ be any metacategory.
A '''commutative square''' in $\CC$ consists of four objects
:$A, B, C, D$
and four morphisms:
:$\alpha : A \to B$
:$\beta  : B \to D$
:$\gamma : A \to C$
:$\delta : C \to D$
such that:
:$\ds \beta \circ \alpha = \delta \circ \gamma$
\end{definition}