Definition 29.49.8. Let $\varphi$ be a rational map between two schemes $X$ and $Y$. We say $\varphi$ is defined in a point $x \in X$ if there exists a representative $(U, f)$ of $\varphi$ with $x \in U$. The domain of definition of $\varphi$ is the set of all points where $\varphi$ is defined.

In your comment you can use Markdown and LaTeX style mathematics (enclose it like $\pi$). A preview option is available if you wish to see how it works out (just click on the eye in the toolbar).