An equivalence of categories is a quadruplet $(F, G, \alpha, \beta)$, where $F : \mathbb{C} o \mathbb{D}$ and $G : \mathbb{D} o \mathbb{C}$ are functors and $\alpha : G F \Rightarrow extrm{id}_\mathbb{C}$ and $\beta : F G \Rightarrow extrm{id}_\mathbb{D}$ are natural isomorphisms.