Natural transformation

https://en.wikipedia.org/wiki/Natural_transformation

Aa natural transformation provides a way of transforming one functor into another while respecting the internal structure (the composition of morphisms) of the categories involved.

Hence, a natural transformation can be considered to be a "morphism of functors". Indeed, this intuition can be formalized to define so-called functor categories.

Natural transformations are, after categories and functors, one of the most fundamental notions of category theory and consequently appear in the majority of its applications.