In my opinion, cheating is "breaking the rules". The rules must be agreed upon by all players. A maphack is cheating unless all players agree to it. Ability hacking is cheating, unless all players agree to it. Sleeping with someone other than your partner is cheating, unless all three people agree to it. On any massive-multiplayer game, any modification is cheating unless they are listed in the game description.
It all depends on who writes the rules. If everyone wants to change the rules, so be it, but if only one or a few change or break the rules, that is cheating against the others. Also, different rule sets must remain separate forever. Just because your character got +138 Plate Armour without breaking rules in that game, doesn't mean you can legally shift them into a game with different rules where that couldn't happen.
