Definition 0.1. Groupoid categories, or categories of groupoids, can be defined simply by considering a groupoid as a category 𝒢₁ with all invertible morphisms, and objects defined by the groupoid class or set of groupoid elements; then, the groupoid category, 𝒢₂, is defined as the 2-category whose objects are 𝒢₁ categories (groupoids), and whose morphisms are functors of 𝒢₁ categories consistent with the definition of groupoid homomorphisms, or in the case of topological groupoids, consistent as well with topological groupoid homeomorphisms. The 2-category of groupoids 𝒢₂, plays a central role in the generalised, categorical Galois theory involving fundamental groupoid functors.

Definition 0.2. Let 𝒢₁ and 𝒢₂ be two groupoids considered as two distinct categories with all invertible morphisms between their objects (or ‘elements’), respectively, x ∈ Ob(𝒢₁) = 𝒢₀¹ and y ∈ Ob(𝒢₂) = 𝒢₀². A groupoid homomorphism is then defined as a functor h : 𝒢 ₁→𝒢₂.

A composition of groupoid homomorphisms is naturally a homomorphism, and natural transformations of groupoid homomorphisms (as defined above by groupoid functors) preserve groupoid structure(s), i.e., both the algebraic and the topological structure of groupoids. Thus, in the case of topological groupoids, G, one also has the associated topological space homeomorphisms that naturally preserve topological structure.