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