\begin{definition}[Definition:Module/Left and Right Modules]
Let $\struct {R, +_R, \times_R}$ be a ring.
Let $\struct {G, +_G}$ be an abelian group.
A '''module over $R$''' is an $R$-algebraic structure with one operation $\struct {G, +_G, \circ}_R$ which is either a left module or a right module:
\end{definition}