Kerodon

Definition 6.1.2.14. Let $\operatorname{\mathcal{C}}$ be a $2$-category and let $f: C \rightarrow D$ and $g: D \rightarrow C$ be $1$-morphisms of $\operatorname{\mathcal{C}}$. We say that a $2$-morphism $\epsilon : f \circ g \Rightarrow \operatorname{id}_{D}$ is the counit of an adjunction if it satisfies the equivalent conditions of Proposition 6.1.2.13: that is, there exists a $2$-morphism $\eta : \operatorname{id}_{C} \Rightarrow g \circ f$ for which the pair $(\eta , \epsilon )$ is an adjunction. If this condition is satisfied, we will say that $\epsilon $ exhibits $f$ as a left adjoint of $g$ and also that $\epsilon $ exhibits $g$ as a right adjoint of $f$.