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I've used "ExP" since about day 1, to avoid confusion with e.p. (electrum pieces, and does anybody know why e.p. no longer seem to exist, or is it that players today just can't handle currency systems where things don't always multiply by ten...)
For some odd reason I've never liked XP as an abbreviation, though.
And I'd never just use X, as X in my game is shorthand for "dead".
I have never seen a poll here or anywhere else with a 100% on one side, and a 0% on the other. Even when only 5 or so people have voted, there's one guy who has to go the other way.
Observations:
(1) I've never, ever heard anyone refer to it as "EP".
(2) In D&D it's officially XP: see last entry in PHB Glossary.
(And all the magic items in DMG.)
Wish we could see who it was, but it's probably safer for that person if we don't know. He's certainly one of the numerous vocal people here else he woulda said something.