A normed (Banach) -bimodule
over normed (Banach) algebras
and
over the same field
or
is a normed space (Banach
space, respectively)
that is simultaneously a normed
(Banach) left A-module and normed (Banach)
right B-module, with
for all
,
, and
. If
, then
is simply called a normed (Banach)
-bimodule. A normed (Banach)
-bimodule is called symmetric or commutative if
for all
,
.
For example, if
is a Banach algebra, then its continuous
dual
can be considered as a Banach
-bimodule with the actions
for
and
.