I know wikipedia isnt that reliable but...
http://en.wikipedia.org/wiki/Pale_ale#Definition
http://en.wikipedia.org/wiki/Pale_ale#Definition
A pale ale has two basic characteristics:
It is an ale, that is, fermented using a top-fermenting yeast.
It is pale, that is, generally between 8 and 14 degrees SRM in colour. While this colour is not "pale" compared to, say, a golden ale or Pilsener, the pale malts used in making pale ale at its inception gave the beer a far lighter colour than the porters common in England at the time.