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.