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Wiktionary
natural transformation

n. (context category theory English) Given categories ''C'' and ''D'' and functors ''F'' and ''G'' both going from ''C'' to ''D'', then a transformation ''η'' from ''F'' to ''G'' consists of components
''η''X:''F''(''X'') → ''G''(''X''), indexed by objects ''X'' of ''C'', which are morphisms in ''D''. The transformation is said to be natural if for any arrow ''f'':''X'' → ''Y'' in ''C'', the following naturality condition is satisfied: eta_Y circ F(f) = G(f) circ eta_X.

Wikipedia
Natural transformation

In category theory, a branch of mathematics, a natural transformation provides a way of transforming one functor into another while respecting the internal structure (i.e. 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.