Definition:Graph (Category Theory)

Definition

A graph is an interpretation of a metagraph within set theory.

Let $\mathfrak U$ be a class of sets.

A metagraph $\GG$ is a graph if and only if:

$(1): \quad$ The objects form a subset $\operatorname{vert} \GG \subseteq \mathfrak U$

$(2): \quad$ The morphisms form a subset $\operatorname{edge} \GG \subseteq \mathfrak U$

If the class $\mathfrak U$ is a set, then morphisms are functions, and the domain and codomain in the definition of a morphism are those familiar from set theory.

If $\mathfrak U$ is a proper class this is not the case, for example the morphisms of $\CC$ need not be functions.