Definition 7.42.1. Let $\mathcal{C}$ be a site. We say an object $U$ of $\mathcal{C}$ is sheaf theoretically empty if $\emptyset ^\# \to h_ U^\#$ is an isomorphism of sheaves.

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).