In category theory, a faithful functor is a functor which is injective when restricted to each set of morphisms with a given source and target.

In other words, a functor F : C → D is faithful if the maps

are injective for every pair of objects X and Y in C.

Note that a faithful functor need not be injective on objects or morphisms. That is, two objects X and X′ may map to the same object in D, and two morphisms f : X → Y and f′ : X′ → Y′ may map to the same morphism in D.

For example, the forgetful functor U : Grp → Set is faithful but neither injective on objects or morphisms.

See also:

• full functor
• subcategory