BAUS: Pronounced "Boss". Beautiful And Unique Snowflake. While every character is unique, some characters are a confusing mishmash of too many confusing plot points which make the character largely indecipherable to anyone but the creator. Your character should be unique and interesting, but no one wants to read a novel to get to know your backstory.
BBEG: Big Bad Evil Guy. Typically the antagonist of the campaign.
Box Text: Pre-written adventures often have text inside of a box for the Gamemaster to read. Sometimes this text is very long, so many players get distracted while box text is being read. Box text is very important, and it is both polite and wise to pay attention when the GM is reading it, even if it is long and sometimes boring.
Freakshow: A half-troll, a catfolk, and an albino demonspawn walk into a bar looking for work. Because this party is a collection monstrous creatures, they are run out of town by scared townsfolk with torches and pitchforks. A Freakshow is a party of creatures with no business among civilized peoples. It is often wise to have at least one normal race in the party to act as the party's Face.
GMPC: Sometimes the GM will write an NPC and make him tag along with the party so that he can both run and play in a game at the same time. While the temptation is understandable, do not do this. Everyone hates it, and it nearly always leads to favoritism, player infighting, and sadness.
Gish: A cross between a martial character and an arcane spellcaster. A Magus is an example of a "Gish".
House Rule: A rule added to the game or changed by the Game Master.
In Character: Conversation or events which takes place inside the game.
Macguffin: An "object" required to complete some objective. The Macguffin can be an object, a person, or a piece of information. "The One Ring" is an example of a Macguffin.
Out of Character: Conversation which takes place between the real players outside of the game.
Player Agency: Absolute control over the player's own character, their personality, and their actions. Player Agency should only be violated in extreme situations.
Rails: The course which the GM imagined the plot would take when he/she wrote the adventure/campaign. Deviating from this plot is often called "going off the rails".
Railroading: A GM who forces the party to follow his plot is "railroading" the party.
Sandbox: A sandbox is a game in which the world is set out before the party, and they are expected to seek out whatever they find interesting. Sandboxes are common in video games, but difficult to manage in tabletop games because writing quality content is typically a veyr time-intensive endeavor.
Save or Die: An effect, such as a spell, which allows a single saving throw to prevent immediate death.
Save or Suck: An effect, such as a spell, which allows a single save to prevent an effect which incapacitates or disables the target.
Supremacy Caster: A spellcaster whose job is to be better than everyone else at everything.
Tank Falacy: Without a video game-style "aggro" mechanic, enemies are often free to disregard your party's front-line martial characters and charge your back-line squishy characters (wizards, etc.). The "Tank Falacy" is the idea that a character is so incredibly difficult to harm ("tanky") that creatures feel no need to reason to attack them when they have comparably frail allies who typically do much more damage. The lesson of the Tank Falacy is that it's fine to be extremely durable, but you need to also be scary offensively or enemies may choose to ignore you in favor of attacking your allies.