What to call the natural transformation in the Yoneda lemma

Question

The Yoneda lemma is an isomorphism between f a and (a <-) ~> f where the Yoneda embedding is Y = (<-). Is there a descriptive name for one particular side of the isomorphism, namely the natural transformation from the Yoneda embedding: Y a ~> f? An alternative to:

hom-set of presheaf homomorphism from the representable presheaf Y a to f

I ask because the Haskell datatype Co.Yoneda f a defines the natural transformation of the covariant Yoneda lemma and it would be useful to have a description that explains its role in the Yoneda lemma.

type    Yoneda :: (Type -> Type) -> Type -> Type
newtype Yoneda f a = Yoneda ((a ->) ~> f)