\begin{definition}[Definition:Affine Open Subset]
Let $\struct {X, \OO_X}$ be a scheme.
Let $U \subset X$ be an open subset.
We say, that $U$ is an '''affine open subset''', if the restriction $\struct {U, \OO_X {\restriction U}}$ of $\struct {X, \OO_X}$ to $U$ is an affine scheme.
Category:Definitions/Algebraic Geometry
Category:Definitions/Schemes
\end{definition}