In Character (IC)
From Tapestries MUCK
In Character, or IC, refers to something that is entirely within the made up world of the characters. Saying a person is IC means that they are Roleplaying their character, responding as the character would instead of themselves. You can also refer to an IC "world" or other things that are part of the character's background or present.
In a muck it is rare that a person is entirely IC, but it does happen in more clear-cut Roleplaying.