This is a constant work in progress that will be added to over time. Got a suggestion? Send it over!
Aggro - The predilection toward violence, most often used to describe NPCs and monsters, but is also used to describe players.
AC - Armor Class.
BBEG - Big Bad Evil Guy (or Gal). The central villain in a plotline. Also sometimes call MainEvilBadGuy, or just the Big Bad.
Buff - A temporary boost, usually from a spell or depletable resource, like a potion.
Challenge Rating (CR) - A measure of how much of a challenge a creature should represent to a party of characters. CR 1 should be roughly equivalent to a party of 4 1st level characters. A lot of variables go into this calculation and it might be a science but art and opinion are the secret sauce that make it work.
COS - Curse of Strahd
Crawl - Short for dungeon crawl.
Crit - Either the best thing in the world, or the worst. In D&D you can have critical successes and critical failures, the meaning of which can depend on house rules as much as core rules.
Disbelieve - An attempt to see past an illusory effect, often also called out to no avail when the players are faced with terrible peril.
DM - Dungeon Master, also known as GM (Game Master), referee or storyteller. THis is the person that hosts and facilitates the world the players play in.
DMG - Dungeon Master's Guide, the primary resource for DMs.
EE - Elemental Evil, a Forgotten Realms storyline built around apocalyptic cults.
EEPC - Elemental Evil Players Companion, a supplement for the Elemental Evil storyline. This supplement is available for free from Wizards of the Coast.
Gish - A fighter (usually melee fighter) with access to magic. The name comes from the name of a Githyanki soldier that does just that
HODQ - Hoard of the Dragon Queen
Homebrew - Rules, settings or accessories created by DMs rather than official Dungeons & Dragons products.
House Rules - Alterations to the core rules in the books to be used in games with a particular DM.
Level - This can refer to a few different things in D&D.
- Character Level - The levels attained by your character gaining experience and used as milestone markers that determine when your character has learned new skills and abilities.
- Dungeon Level - Often "dungeons" are set up in "levels" with the first level being easiest and the last level having the BBEG.
- Spell Level - Spells are categorized in levels that have very little to do with character levels. Oh sure, your character has to be a certain level to be able to access higher spell levels but it's seemingly arbitrary. You would think a 5th-level spellcaster would be able to use 5th level spells, but no, that is really never the case.
- Effective Character Level (ECL) - This is used to describe Monsters and NPCs. The calculation was "Racial hit dice + character level + Level adjustment" with the idea being that a creature with an ECL of 10 would be roughly equivalent to a 10th level character. ECL has been dropped in 5th Edition.
MM - Monster Manual
Murder Hobos - A pretty good description of most parties in D&D and other RPGs who tend to be essentially homeless people running around murdering other people and races and looting their corpses.
OP - Overpowered, or Original Poster in threaded online discussions.
Passive Perception - Things you notice without trying. This is mostly used in checking a skill versus an unaware target. For example, as a rogue, you might be trying to sneak up to a bored guard. The guard isn't actively looking for you but has a chance of notice you anyway. Some creatures have a much higher passive perception due to natural or keen senses. For example, a dragon might have a chance to smell or hear you even if it isn't actively looking for you.
Perception - A skill modified by wisdom score that represents how aware a character is of the surrounding. Often, the DM will have players roll a perception check to see if they notice something, like a rogue hiding in the shadows, a slip of paper on the ground or a wyvern circling above.
RAI/ROI - Rules As Intended, as in the intent of the rule, even though the literal written rule may not convey it.
RAW - Rules As Written, as in the specific way the rules were written, as opposed to RAI.
Rule of Cool - Originally a TV Trope, this was stated as "The limit of the Willing Suspension of Disbelief for a given element is directly proportional to its awesomeness." In game terms, it is usually applied to a DM allowing a player to bend the rules or use them creatively in a way that isn't RAI to have something cool happen. Often, it's just for adding cinematic effect to a scene, sometimes it has to do with rewarding creativity, other times it's just because something is hilarious.
SCAG - Sword Coast Adventures Guide
Squishy - A description of players who are fragile in some way, usually lacking armor and having low Hit points.
Toon - A term from some MMORPGs, referring to their character/avatar/whatever. Often discouraged at table-top games.
TPK - Total Party Kill
UA - Uneather Arcana
WoTC - Wizards of the Coast, the game company that owns and releases Dungeon & Dragons materials.
XP - Short for experience
Give me ideas of other terms to add here.