The story
Lemma named after the Japanese mathematician Nobuo Yoneda (1930–1996).
Etymology from Wiktionary, CC BY-SA 4.0.
Yoneda lemma
Today it means: Given a category 𝒞 with an object A, let H be a hom functor represented by A, and let F be any functor (not necessarily representable) from 𝒞 to Sets, then there is a natural isomorphism between Nat(H,F), the set of natural transformations from H to F, and the set F(A). (Any natural transformation α from H to F is determined by what α_A( mbox id_A) is.) Full definition →