\begin{definition}[Definition:Real Function/Two Variables]
Let $S, T \subseteq \R$ be subsets of the set of real numbers $\R$.
Let $f: S \times T \to \R$ be a mapping.
Then $f$ is defined as a '''(real) function of two (independent) variables'''. 
The expression:
:$z = \map f {x, y}$
means:
:(The dependent variable) $z$ is a function of (the independent variables) $x$ and $y$.
\end{definition}