THAC0
To Hit Armor Class 0
Example
I'm so glad I never played DnD while THAC0 was a thing. It sounds miserable
THAC0 definition by Slang.net
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Those well-versed in obscure Dungeons & Dragons (DnD) rules know that THAC0 stands for To Hit Armor Class 0. This acronym refers to a complicated and (thankfully) obsolete system by which players previously determined whether they'd hit an opponent.
How did THAC0 work?
In some previous versions of DnD (most notably, AD&D Second Edition) characters' armor class (AC) actually decreased as they donned better armor. (Yes, that's extremely unintuitive.) Additionally, each character (PC and NPC alike) had a set of THAC0 scores, which determined what number they needed to roll on a d20 to hit an opponent with an AC of 0. A character's THAC0 score decreased as they leveled up, making it easier to hit opponents.
To determine whether they hit an opponent, a player would roll a d20, note their THAC0 score, subtract the opponent's AC from their THAC0 score, and then compare the result of their roll to the modified THAC0 score. If the result of the roll was equal to or higher than the modified THAC0 score, the player hit. (Alternatively, players would just consult a comprehensive attack matrix that showed them the number they needed to hit, without doing any math.)
Is THAC0 still used in DnD?
To most players' delight, DnD Third Edition jettisoned the THAC0 system. These days, DnD combat math is much more straightforward, and most players will never need to understand what THAC0 means or how it worked. However, some old-school players enjoy busting out their knowledge of THAC0 to prove their geek cred.