Self-dual describes an object 
that is equal or isomorphic
to its own dual, according to the notion of duality in use. A number of areas of
mathematics have a notion of duality that can be applied to their objects.
For example, any normed space has a dual normed space. Hilbert spaces are self-dual normed spaces (up to isomorphism of Hilbert spaces).
A geometric proposition is said to be self-dual when application of the duality principle of projective geometry results in a proposition equivalent to the original. Desargues' theorem is an example of a self-dual proposition.
Other examples of self-dual mathematical objects include self-dual configurations, self-dual graphs, self-dual polyhedra, and self-dual codes.
