Abstract Algebra
《Abstract Algebra》 · 第 847 页 · Section 18.1「Linear Actions and Modules over Group Rings」，Definition (2)
Let $R$ be a ring and let $M$ be a nonzero $R$-module. The module $M$ is said to be indecomposable if $M$ cannot be written as $M_1\oplus M_2$ for any nonzero submodules $M_1$ and $M_2$; otherwise $M$ is called decomposable.