Page "Natural transformation" Paragraph 24

This is formally the tensor-hom adjunction, and is an archetypal example of a pair of adjoint functors.

Natural transformations arise frequently in conjunction with adjoint functors, and indeed, adjoint functors are defined by a certain natural isomorphism.

Additionally, every pair of adjoint functors comes equipped with two natural transformations ( generally not isomorphisms ) called the unit and counit.