Fortunately or unfortunately, there are a lot of definitions. I've even heard it said that there are as many definitions as there are permacultur-ists, because permaculture is different everywhere you go. Like Nature.
The word can get broken down into perma, from permanent, and culture, from agriculture or even just culture itself, because lets face it there will never be any culture without food to feed the people of that culture.
Mother Nature has worked for millions of years to create a system that provides more energy than it consumes, so many use the logic that if we are going to create permaculture too, we should use the examples set before us for how to do it best. To me that seems the most efficient, especially considering that many times we have seen ourselves implementing new technology that has unintended consequences and requires even more work in the long run (like having to pollinate our own produce because we have killed all of the bees.)
According to Wikipedia, "Permaculture is a set of design principles centered on whole systems thinking, simulating, or directly utilizing the patterns and resilient features observed in natural ecosystems. It uses these principles in a growing number of fields from regenerative agriculture, re-wilding, and community resilience."
I want to add that permaculture concentrates on connecting pieces of an environment so that all waste is used by another element of the design, again just like in nature. Permaculture is waste-free, resilient, and requires less work to produce a yield over time. In terms of harvest quantity, permaculture gardens are known to provide more variety, over a longer period, and like I said with no waste and less work. So why don't more people do this!? Because we haven't been taught.
Picture credit goes to https://commons.wikimedia.org/wiki/File:Raised_bed.jpg