 A morphism "Y"formula_23 "X" of stacks is representable if, for every morphism "S" formula_23 "X" from (the stack associated to) a scheme to X, the fiber product "Y" ×"X" "S" is isomorphic to (the stack associated to) an algebraic space.The fiber product of stacks is defined using the usual universal property, and changing the requirement that diagrams commute to the requirement that they 2-commute.See also morphism of algebraic stacks for further information.