Basic Category Theory
《Basic Category Theory》 · 第 59 页 · Definition 2.3.1
Given \(S:\mathcal A\to\mathcal C\) and \(T:\mathcal B\to\mathcal C\), the comma category \((S\downarrow T)\) has objects \((A,B,h)\) with \(h:S(A)\to T(B)\), and morphisms the pairs making the evident square commute.