In Character
From NSwiki, the NationStates encyclopedia.
IC is the Roleplaying acronym for In-Character. The adjective/adverbal form is ICly. It represents information and actions known or performed by characters in a given storyline or roleplay. As such, this information or these actions would be known to any other characters in a position to notice them.
The converse is OOC, or Out-of-Character, which theoretically has no plotline effect (see Metagaming).