My definition is:
Permaculture: A philosophy of reducing ongoing inputs.
The wikipedia definition for philosophy is currently "the study of general and fundamental problems" which seems to be of appropriate scope for permaculture. The words "reducing" is used instead of "eliminating" or "sustainable" as a recognition that permaculture is more of a process than a destination. It is also a recognition that nothing is sustainable and some inputs will always be needed. "Ongoing inputs"
should not be confused with "initial inputs" or "start-up inputs" as permaculture systems often require substantial start-up inputs in order to decrease the ongoing inputs that regularly occur once a system has reached maturity.