Categories for the Working Mathematician
《Categories for the Working Mathematician》 · 第 65—66 页 · Chapter III, Section 3
The pushout of arrows \(f:a\to b\) and \(g:a\to c\) is a universal commutative square with vertex \(r\): every compatible pair \(u:b\to s\), \(v:c\to s\) factors uniquely through \(r\).