Well, it's chronological by definition, but there's an aesthetic there too. I always thought of it as those late 70s/early 80s bands that might have started out as punk but soon developed a more experimental sound, usually one you could describe as dark, atmospherically. Bands like Joy Division, the Fall, Killing Joke, the Birthday Party, maybe stuff like Cabaret Voltaire and Throbbing Gristle. I see the Wiki entry has a far broader definition.