I find myself asking that question and occasionally having to answer it when other people ask. So I should write and answer, perhaps one about what it means to me as even Holmgren writes:
"The quest by some for a completely consistent and logical picture of permaculture may not be useful. Rather than seeking to define or control permaculture, I write about it as simply one more contribution to understanding, meaning and action in a world full of uncertainty." --- Holmgren, PPPBS, pg. 9. "The Purpose of this book"
Perhaps permaculture is three things:
Permaculture is a creative design response grounded in systems thinking to realise a progression towards a world of declining reliance upon energy.
There are a few components to this definition, I should elaborate on these at some point:
"Permaculture is a creative design response to a world of declining energy" --- Holmgren, PPPBS,