Let be a normed (Banach) algebra. An algebraic left -module is said to be a normed (Banach) left -module if is a normed (Banach) space and the outer multiplication is jointly continuous, i.e., if there is a nonnegative number such that . If has an identity , then is called unital if for all . A normed (Banach) right module can be similarly defined.
For example, every closed left ideal of a normed algebra can be regarded as a Banach left -module with the product of giving the module multiplication.