An Introduction to Homological Algebra
《An Introduction to Homological Algebra》 · 第 325 页 · Section 6.1, Definition
If \(\mathcal A\) is \(R\text{-}\mathbf{Mod}\) or \(\mathbf{Mod}\text{-}R\), then a free resolution of a module \(A\) is a projective resolution in which each \(P_n\) is free.