Definition 29.22.1. Let $f : X \to S$ be a morphism.

1. We say $f$ is open if the map on underlying topological spaces is open.

2. We say $f$ is universally open if for any morphism of schemes $S' \to S$ the base change $f' : X_{S'} \to S'$ is open.

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).