IC Stands for "In Character".
IC is a concept that defines basic role-play. It is the state of being when you are actively playing your role as a character in this world.
This means that you take actions only as your character would take, and say things only as they would say. It is a made up personality that you create and represent in this world. It is not your real personality, or character as exists in the real world. When you are in character, there are certain things that are inappropriate.
For example, asking for information related to the game over the channels is not accepted. The channels are considered "Out of Character". Things such as the location of another character or how to solve a quest are considered IN CHARACTER and should be discovered by your character within the context of the game.
Things such as sending mudmail to another player is considered in character so this is acceptable.
See Also: OOC