Bertie Russell said philosophy was the No Man's Land between theology and science, maybe a good def. of post-punk would be the No Man's Land between punk and new wave. I try not to use the term in a way that is anomalous to the way I've seen musicians or writers employ it.
Wikipedia's post-punk section is almost as big as its section on Thundercats or Jem. "Post-rock" has a big section, too.
-B.
Chris Doss wrote:
"It's like punk, only with fewer power cords and more self-reflective angst. What Sartre was to Heidegger, so postpunk was to punk!"