Out of Character

A Sub-Trope of Canon Defilement.

In Derivative Works (especially Fan Fiction), this term means that somebody is acting largely against his or her established personality. How and why this occurs has a pretty wide range:

Perhaps the writer simply doesn't understand what he's writing about.
Perhaps he is applying his own Alternate Character Interpretation to them (of which, both Possession Sue, Ho Yay, Foe Yay, and Die for Our Ship contributes towards).
Perhaps he's just writing a Transplanted Character Fic. Regardless, it's usually frowned upon to post stuff not labeled as such, with the term becoming quite negative if it gets applied to a story by other people.
If the Fanfic is of the 10%, this is likely because something in the story's backstory or plotline made them act this way.

Generally viewed as a very negative trait (if a fanfic gets tagged as "OOC", it's usually not a good sign, though writers will often be honest enough to slap the tag on themselves up front).

Compare Character Derailment, which is this applied to canon. See also Out-of-Character Moment. Draco in Leather Pants, Ron the Death Eater and Wimpification can be considered subtropes.

Not to be confused with In and Out of Character. In Role-Playing Games, it is sometimes necessary to make a distinction between when a player is In Character and Out Of Character, to know if the person is speaking as the character or as the player. A player who uses Out of Character information (such as the presence of goblins in a room ahead) to make an in-character choice is said to be MetaGaming, which most dungeon-masters severely frown upon.