Definition:Affine Open Subset

Definition
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.