We have an isomorphism of categories[1]
via the delooping functor $\mathsf{B}\colon \mathsf{Mon}\to \mathsf{Cats}$ of of , exhibiting monoids as exactly those categories having a single object.Proof of Example 8.1.2.1.2.
Omitted.
Footnotes
[1] This can be enhanced to an isomorphism of $2$-categories between the discrete $2$-category $\mathsf{Mon}_{\mathsf{2disc}}$ on $\mathsf{Mon}$ and the $2$-category of pointed categories with one object.