The group algebra , where is a field and a group with the operation , is the set of all linear combinations of finitely many elements of with coefficients in , hence of all elements of the form
(1)

where and for all . This element can be denoted in general by
(2)

where it is assumed that for all but finitely many elements of .
is an algebra over with respect to the addition defined by the rule
(3)

the product by a scalar given by
(4)

and the multiplication
(5)

From this definition, it follows that the identity element of is the unit of , and that is commutative iff is an Abelian group.
If the field is replaced by a unit ring , the addition and the multiplication defined above yield the group ring .
If , and is the usual addition of integers, the group ring is isomorphic to the ring formed by all sums
(6)

where are integers, and for all indices .
Let be a locally compact group and be a left invariant Haar measure on . Then the Banach space under the product given by the convolution for is a commutative Banach algebra that is called the group algebra of .