The Stacks project

Definition 64.18.1. Let $X$ be a Noetherian scheme. A $\mathbf{Z}_\ell $-sheaf on $X$, or simply an $\ell $-adic sheaf $\mathcal{F}$ is an inverse system $\left\{ \mathcal{F}_ n\right\} _{n\geq 1}$ where

1. $\mathcal{F}_ n$ is a constructible $\mathbf{Z}/\ell ^ n\mathbf{Z}$-module on $X_{\acute{e}tale}$, and

2. the transition maps $\mathcal{F}_{n+1}\to \mathcal{F}_ n$ induce isomorphisms $\mathcal{F}_{n+1} \otimes _{\mathbf{Z}/\ell ^{n+1}\mathbf{Z}} \mathbf{Z}/\ell ^ n\mathbf{Z} \cong \mathcal{F}_ n$.

We say that $\mathcal{F}$ is lisse if each $\mathcal{F}_ n$ is locally constant. A morphism of such is merely a morphism of inverse systems.