Any system of ethics that is based around the inner legislation (or rational consideration) of the individual. The word was used by the philosopher Immanuel Kant in his work, "The Metaphysical Elements of Ethics", as an alternative to eudaemonism.
To see its usage in its original context, browse a full copy of Kant's work online at http://eserver.org/philosophy/kant/metaphys-elements-of-ethics.txt.