Natural Transformation

A natural transformation between a pair of Functors $F,G:\mathcal{C}\to\mathcal{D}$ consists of a family of morphisms $\eta_{X}:\mathcal{D}(F(X),G(X))$ that satisfy the following naturality condition:

• $\forall f:\mathcal{C}(X,Y).\;\eta_{Y}\circ F(f)=G(f)\circ\eta_{X}$