Let $\mathcal{G} = (\mathcal{G}_1 \stackrel{\to}{\to} \mathcal{G})$ be a groupoid. A module over the groupoid$\mathcal{G}$ is a collection $\{N_x\}_{x \in \mathcal{G}_0}$ of abelian groups equipped with a collection of maps

$N_x \times \mathcal{G}(x,y) \to N_y$

that are linear and respect the groupoid composition in the obvious way.