The following is a glossary of roleplaying terms that can be quite confusing when you start off. If any are missing, please let me know and I'll add accordingly!

3DMG - three-dimensional mobility gear, the technology used by the military to fight Titans and scale the Walls.
admin/administrator - the owner of the forum, who presides over everything: Puffin.
app - 'application', the character biography.
archive/Archives - the forum that houses finished or inactive threads. Also used in the context of 'this can be archived' when requesting a finished thread be moved to the Archives.
avatar/avi - the image next to people's names.
AU - 'alternative universe', different from following the main story of the anime/manga.
canon - the 'canon' is the source: the anime or manga. So 'in canon' means 'in the anime/manga'.
canon character - a 'canon' or 'canon character' is a character in the anime/manga and thus not originally created for roleplay alone.
character development - the development of a character, where you see how they have changed over time and due to experiences they've gone through.
Discord - a chatting service on which you can create servers and directly message others.
FC/face claim - the character or artwork used to represent a character's appearance.
forum/sub-forum - alternative words for 'board' or 'child board'.
godmoder/godmoding - somebody who godmods, metagames, powerplays, or retcons. (See 'post etiquette' for details)
godmodding - creating an unrealistically strong character or a character that dodges (almost) every attack and lands all of their attacks.
IC - in-character.
metagame/metagaming - the act of using out-of-character knowledge within roleplay, while the character would not be aware of the information.
mod/moderator - the site's staff who assist the admin in looking after the site and help members.
MP - 'Military Police'.
NPC - 'non-playable character', a character not apped or written by anybody in particular.
OC - original character, one made up and not in the actual anime/manga.
ODM gear omni-directional mobility gear. The anime version of 3DMG.
OOC - out-of-character.
open - a roleplay thread that is open for anybody to reply to, provided it fits their character to be there.
playby - the character or artwork used to represent a character's appearance.
plotter - a topic focused on different plots somebody wants for a particular character.
post - often used in context of 'I have so many posts to do!' or 'I need to catch up on posts'; this means 'replies'.
powerplay/powerplaying - controlling another person's character without permission.
rapidfire - posting replies in one or more threads consecutively, often over a few hours time or less.
Recon Corps - another name for the Survey Corps/Scouting Legion.
retcon/retconning - erasing or editing events that have already happened, especially without permission from others involved.
reply - a reply to a thread, also uses in context of 'I have so many replies to do!' or 'I need to catch up on replies'.
roleplayer - essentially 'writer'.
SC/SL - 'Survey Corps' and 'Scouting Legion' respectively.
Scout - a member of the Survey Corps/Scouting Legion.
Shifter - a Titan Shifter; a human who can turn into a Titan, often with unique appearances or powers.
signature - the little space at the bottom of posts, which gives either an image/GIF or some writing. Can be changed within profile settings.
snipe/sniping - joking term for replying to somebody within an hour or within 24 hours (depending on usual thread speed) after their last post.
starter - a starting post or topic.
Stationary Guard - another name for the Garrison Regiment.
thread - a roleplay topic.
tracker - a topic that lists all the threads a character has been in, usually in chronological order or categorised by which characters they've interacted with.
VME - 'vertical maneuvering equipment', similar to 'ODM gear' and '3DMG'.
volley/volleying - quickly posting in one same thread over the course of a day.
wanted ad - a 'wanted' is a plot or character created with a connection to existing characters, and is advertised for others to take up.
WIP - 'work in progress'.
WoF/Wings of Freedom - the symbol of the Survey Corps/Scouting Legion, also used as an alternate name for them.