When used in reference to MMORPG's (Massive Multi-Player Online Role Playing Games), such as World of Warcraft or Everquest, refers to a person who creates many different characters, rather than just one or two. May also be used to refer to a player who creates characters on many different servers of the same game, where the experiences they see will often be different from each other.
Use of the word is intended to be comical in nature, a play on the word "alcoholic" and will often garner laughter from others if used correctly.
Often only used online, and usually only on chat forums.
"I have over twenty characters spread out over five servers! I'm such an altoholic!"