\begin{definition}[Definition:Operation/Binary Operation]
A binary operation is the special case of an operation where the operation has exactly two operands.
A '''binary operation''' is a mapping $\circ$ from the Cartesian product of two sets $S \times T$ to a universe $\mathbb U$:
:$\circ: S \times T \to \mathbb U: \map \circ {s, t} = y \in \mathbb U$
If $S = T$, then $\circ$ can be referred to as a '''binary operation on''' $S$.
\end{definition}