In algebraic geometry, given algebraic stacks p : X → C, q : Y → C {\displaystyle p:X\to C,\,q:Y\to C} over a base category C, a morphism f : X → Y {\displaystyle f:X\to Y} of algebraic stacks is a functor such that q ∘ f = p {\displaystyle q\circ f=p}. Wikipedia