Suppose that
and
are two normed (Banach) algebras. A vector space
is called an
-bimodule whenever it is simultaneously a normed (Banach)
left
-module,
a normed (Banach) right
-module, and
. If
, then
is simply said normed (Banach)
-bimodule. A normed (Banach)
-bimodule is called symmetric or commutative if
for all
,
.
For example, if
is a Banach algebra, then its dual
can be considered as a Banach
-bimodule with the actions