Basic Category Theory
《Basic Category Theory》 · 第 28 页 · Definition 1.3.1
For functors \(F,G:\mathcal A\to\mathcal B\), a natural transformation \(\alpha:F\to G\) assigns a map \(\alpha_A:F(A)\to G(A)\) to each object \(A\), such that \(G(f)\circ\alpha_A=\alpha_{A'}\circ F(f)\) for every \(f:A\to A'\).