A C-algebra is a complex Banach algebra together with an isometric antilinear involution satisfying (a b) = b* a* and the C-identity ‖ a a ‖ = ‖ a ‖2.
For bounded operators on a given Hilbert space, C-algebras characterize topologically closed subalgebras of ${\mathcal B}({\mathcal H})$ (in operator norm), also closed under taking the adjoint operator. C-algebras are at the heart of noncommutative-geometry and are extensively used in mathematical-physics.