The other day I ordered an Eagle Rock Brewery XPA on tap. At the time, I had no idea what that was, but according to the brewer's site:
Extra Pale Ale (XPA) is a variation of American pale ale – bright and refreshing with a substantial hop presence.
I'm guessing the thing that is "extra" is the paleness. To my (very) untrained palate, it tasted pretty much like any other IPA/APA I've ordered. Maybe it was a bit more bitter though?
Is there any standard that differentiates an XPA from other American pale ales?