Definition of Crack
Crack: A work of fanfiction (or art or video) that has an absurd, surprising or ridiculous element to it. One that is intentionally unbelievable and insane, and often humorous. Usually short one-shots, simple vids or illustrations. The oddity can be a pairing, a situation, or an all-around AU. That is not to say it needs to be ludicrous, either - it can have serious elements to it (i.e. plot!), it merely needs some sort of twist on our "normal" reality.