The acronym IC stands for "In Character".
Basically whenever you are role-playing you are "In Character". It is a good thing to note that when a person acts like a jerk or starts flirting with you, it may have nothing to do with the actual person behind the avatar, just how the character would act. It is wise not to become offended by anything someone does "In Character".
Also shorthand for "I See". (Most likely not on a role playing server)
An acronym for If I Remember/Recall Correctly.
An acronym for In my honest/humble opinion. IMO can also be used, being just in my opinion.
Playing inside an instance.
An acronym for Internet relay chat. Check it AoCWiki:IRC for more information about how to chat with other AoCWikians!
An acronym for In real life, meaning life outside the game such as work, family etc.
Imba is an abbreviation for imbalanced that is often used within the realm of online gaming (especially on online message boards). It generally refers to a tactic or ability that is considered unfairly powerful.
InstaGib refers to the act of killing a monster or player in one shot. This is usually considered to be imba.
An abbreviation for Intelligence, a character attribute.
Intelligence provides your character with mana and increases the magic rating for mages.
An abbreviation for Inventory, usually the backpack you carry around with you ingame.
An abbreviation for Invisible.
An abbreviation for Invulnerable.
Typically refers to a character skill or item which is imba by being so powerful, when it's used, the player using the skill/item almost always wins the conflict.