Definition 6.1.2.10. 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 $\eta : \operatorname{id}_{C} \Rightarrow g \circ f$ *is the unit of an adjunction* if it satisfies the equivalent conditions of Proposition 6.1.2.9: that is, if there exists a $2$-morphism $\epsilon : f \circ g \Rightarrow \operatorname{id}_{D}$ for which the pair $(\eta , \epsilon )$ is an adjunction. If this condition is satisfied, we will say that $\eta $ *exhibits $f$ as a left adjoint of $g$* and also that $\eta $ *exhibits $g$ as a right adjoint of $f$*.

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