\begin{definition}[Definition:Compatible Module Structures/Definition 3]
Let $A$ and $B$ be rings.
Let $\struct {M, +}$ be an abelian group.
Let $* : A \times M \to M$ and $\circledast: B \times M \to M$ be left or right linear ring actions so that:
:$(1): \quad \struct {M, +, *}$ is a left or right module over $A$
:$(2): \quad \struct {M, +, \circledast}$ is a left or right module over $B$
The module structures are '''compatible''' {{iff}} for all $b \in A$, the homothety $h_b : M \to M$ is an endomorphism of the $A$-module $M$.
That is, {{iff}} the image of the ring representation $B \to \map {\operatorname {End} } M$ is contained in the endomorphism ring $\map {\operatorname {End}_A} M$.
\end{definition}