Permaculture is a branch of ecological design, ecological engineering, environmental design, construction and Integrated Water Resources Management that develops sustainable architecture, regenerative and self-maintained habitat and agricultural systems modelled from natural ecosystems. The term permaculture (as a systematic method) was first coined by Australians Bill Mollison and David Holmgren in 1978. The word permaculture originally referred to “permanent agriculture” but was expanded to stand also for “permanent culture,” as it was seen that social aspects were integral to a truly sustainable system as inspired by Masanobu Fukuoka’s natural farming philosophy.
Permaculture is an ethically based design science that is used to develop sustainable/regenerative landscapes and human based systems. Often energy passes through a system and slowly loses its quality and thereby losing its ability to drive systems. This is entropy.
Ecosystems are very very efficient at capturing, storing and using this quality from energy as it cycles it through the complex relationships within ecosystems. This is the basis for permaculture design, you are capturing, storing and using this quality energy before it leaves and eventually dies (entropy) Our job as a designer is to cycle energy in as many ways as possible through a complexity of interdependent relationships.
This is made possible through design, through the assembling of interactions which form the systems you implement. The deeper you go the more you understand and then everything manifests itself together as a whole.
Regenerative is basically a system that is improving. Improving from an ecological perspective is increasing in diversity, complexity of interactions or moving from simple to complex. Because of this as an example, soil is being built, biomass is increasing which in turn is sequestering more carbon, building more soil biology, this is an example of a regenerative system. Bare soil moving to a forested state is behaving regeneratively ( until it stabilizes and starts to decrease in biodiversity which in turn reduces interactions etc ) This is why disturbance is a natural, necessary process in ecosystems as it knocks it back to a more complex phase.
Permaculture is a design tool to be used, and can only be sustainable if used correctly. You cannot 'do permaculture ' you can only use it.