Algebraic Geometry
《Algebraic Geometry》 · 第 109 页 · Chapter II，Section 5，Definitions
An $\mathcal O_X$-module $\mathcal F$ is free if it is isomorphic to a direct sum of copies of $\mathcal O_X$. It is locally free if $X$ can be covered by open sets $U$ for which $\mathcal F|_U$ is a free $\mathcal O_X|_U$-module. A locally free sheaf of rank $1$ is also called an invertible sheaf.