The Stacks project

Definition 4.29.2. Let $\mathcal{C}$ be a $2$-category. A sub $2$-category $\mathcal{C}'$ of $\mathcal{C}$, is given by a subset $\mathop{\mathrm{Ob}}\nolimits (\mathcal{C}')$ of $\mathop{\mathrm{Ob}}\nolimits (\mathcal{C})$ and sub categories $\mathop{\mathrm{Mor}}\nolimits _{\mathcal{C}'}(x, y)$ of the categories $\mathop{\mathrm{Mor}}\nolimits _\mathcal {C}(x, y)$ for all $x, y \in \mathop{\mathrm{Ob}}\nolimits (\mathcal{C}')$ such that these, together with the operations $\circ$ (composition $1$-morphisms), $\circ$ (vertical composition $2$-morphisms), and $\star$ (horizontal composition) form a $2$-category.