It is quite common that the question: 'What is Permaculture?', is answered by rather long and vage monologue ending in: 'Yeah, well just get yourself a PDC, than you will know'.
It might be so difficult to catch in a single line because its such a diverse tool, that you really need to dig deep to get to the core of it. In the 2010 documentary: 'Permaculture: The Growing Edge', it is stated as:
'Permaculture is the art of designing beneficial relationships'.