Alternative Universe (AU): This is a term for a universe that deliberately deviates from the canon. Can be a minor deviation, or a large one. Or something in between.
Backchannel(ing): The practice of chatting and plotting in private e-mail, messages or IM, instead of public view on the forum. Example: "It's okay for Joe's player to say that my character is there. I said it was okay in the backchannel."
Canon: The original source material, e.g. the book, manga, cartoon, TV show, video game, etc. where the characters and/or world came from.
Closed Thread(s): A thread that has been marked as being for a particular character/group of characters.
Crack: Something that is intentionally humorous or parody in nature. Not to be taken seriously – e.g. Lord Voldemort hosting a cooking show.
Fade to Black: The ending of a scene as written by the players, but when IC action is presumed to continue – sometimes the thread picks up again after a certain point. Is most generally used when characters are about to have sex, although it can be used for other reasons.
Fanon: Something that has been used by fans for so long or so much that it is generally accepted as almost as true as canon.
Gary-Stu: Male version of a Mary-Sue
Godmoding (godmoding, moding, godmodding etc.): Where a character does something they are not capable of/do not have permission to do to another character, e.g. moving another character (without actual physical intervention by the godmoding character) by stating they are somewhere they are not without the other player's permission.
IC Actions = IC Consequences (ICA=ICC): What happens after a character makes an action, be it good (helping a stranger and being rewarded) or bad (mouthing off to a teacher and getting punished). A sign of Mary-Sueing is twisting things around to avoid the negative side of ICA=ICC.
IC/OOC Line: The line between 'in character' and 'out of character'. Players are not their characters, are not necessarily even like their characters, and vice versa.
In Character (IC): Anything that occurs in/relates to what happens in the game (to/by the characters).
Mary-Sue (Sue): A (female) character that defies the 'rules' of the setting, is incredibly accomplished for no reason, can do things that no other can reasonably do, seeks to make themselves the focus etc.
Metagaming: Bringing OOC knowledge into an IC situation, e.g. knowing that a character really likes ice cream, when it has never been mentioned, and your character has no basis for knowing this fact.
Non-Joined Character (NJC): A character that is more fully developed than an NPC, but belongs to one player and is most likely recurring for a plot/specific events. Examples include spouses and children of PCs, who make appearances but are not the focus.
Non-Player Character (NPC): A character that is not important to be created, able to be used by any player, and generally only for a situation e.g. the waitress serving a group at dinner, the student a person bumps into while waiting in line.
Open Thread(s): A thread that is open to any character that, Icly, has a reason to be there.
Original Canon Character (OCC): Not quite the same as an OC, but very similar. A character who has a basis in canon, but is little more than a name (often only mentioned), thus requiring the player to build on that framework extensively. Examples include Astoria Greengrass (mother of Scorpius Malfoy), Septima Vector (Arithmancy professor) and Aurora Sinistra (Astronomy professor).
Original Character (OC): A character who was created does not appear in the canon source, but was created from scratch by a player.
Out of Character (OOC): Anything that occurs/relates to things out of the game, e.g. OOC knowledge (you know a character is evil, but your own character does not).
PB: The “face” used to represent a character visually; akin to “casting” characters for a movie. May be real people, drawings, or other.
Retcon: Short for 'retroactive continuity'. This refers to a player (or players) OOCly backtracking and erasing plots from the IC continuity as if they had never happened.
Role-Playing (role-play, RP, RPing): The concept where one plays games with others by assuming a 'role'.
Tags/Tagging: The concept of “marking” a thread as either open (available to all) or closed (meant for particular characters).
Wangst: When angst goes bad: either overboard in its severity (as though things were just added for the 'drama factor') - 'My father ate my bunny rabbit because he didn't like me!' - and/or is the continual focus of the character (every thread is designed to showcase the horror of this character's past). There is no attempt to 'deal with' their problems, or any sign of 'improvement' or character development beyond things simply getting worse. While some people do wangst over things, it is usually limited to a specific period of time, before they move onto the next stage (whatever that may be). A little angst can be good, but angst for the sake of angst (and dra-ma!) is simply wangst.