From the originators:

“Permaculture is a philosophy of working with, rather than against nature; of protracted and thoughtful observation rather than protracted and thoughtless labour; and of looking at plants and animals in all their functions, rather than treating any area as a single product system.”
Bill Mollison

“Permaculture is defined as consciously designed landscapes which mimic the patterns and relationships found in nature, while yielding an abundance of food, fibre and energy for the provision of local needs…more precisely I see Permaculture as the use of systems thinking and design principles that provide the organising framework for implementing the above vision”
David Holmgren ‘Pathways to Sustainability’ 2004

And subsequently:

‘....a design system that supplies all our needs while giving us a sustainable outcome.’
Geoff Lawton, where “sustainable” = a system that:
- Meets its own needs for growth and maintenance
- Produces a surplus for re-investment (Mollison)

Actually, the word sustainable has fallen into disrepute nowadays, having been usurped and applied, for example, to “sustainable economy” by the likes of Tony Blair, to describe the same economic system that got us into all this trouble, and how to keep it going. Nowadays the term “regenerate” and “regenerative” is more liked as it describes not just a system that keeps going but one that improves.

“A revolution dressed up as gardening”
Mike Feingold - to describe how permaculture is often thought of as a gardening technique but is in fact much broader in its application, but that’s OK if it gives people an entry point, they’ll soon find out that it’s much more!

Hence
"Permaculture has, in many people's minds, come to represent a sustainable, organic, home vegetable garden." (Rosemary Morrow)

One of my favourites is:
“Permaculture is the conscious application of the principles of ecology to the design of sustainable (regenerative) human habitats” (Lea Harrison)

And another by the late and great Chuck Marsh:
“Permaculture is a plot developed by the plants and animals to heal the Earth by instilling in humans a level of caring that has had a few billion years of experience and evolution behind it”

One of Looby’s favourites is:
“A way of thinking differently” This idea led Looby to write her second book 7 Waus to Think Differently, as a way of answering this question - what is permaculture? and to distill and synthesise the shifts in thinking that come about through permaculture training.