Abstract Algebra
《Abstract Algebra》 · 第 351 页 · Section 10.3「Generation of Modules, Direct Sums, and Free Modules」，Definition (4)
A submodule $N$ of $M$ (possibly $N=M$) is cyclic if there exists an element $a\in M$ such that $N=Ra$, that is, if $N$ is generated by one element: $N=Ra=\{ra\mid r\in R\}$.