In the paper titled:
I found the following definition:
Let $k$ be a commutative ring with an identity element, let $R$ be a $k$-algebra (not necessary with an identity element). An additive abelian group $M$ is called a left $R/k$-module if it is a left R-module and a unitary $k$-module satisfying $a(xm) = (ax)m = x(am)$ for all $m \in M$, $x \in R$ and $a \in k$.
I search a reference (i.e. book) where I can find the definition of this kind of modules.
Thanks.