\begin{definition}[Definition:Quasi-Separated Morphism of Schemes]
Let $\struct {X, \OO_X}$ and $\struct {Y, \OO_Y}$ be schemes.
Let $f : \struct {X, \OO_X} \to \struct {Y, \OO_Y}$ be a morphism of schemes.
$f$ is '''quasi-separated''' {{iff}} the diagonal morphism $\Delta_f : X \to X \times_Y X$ is quasi-compact.
\end{definition}