THAC0

THAC0 stands for To Hit Armor Class 0 (zero). In 2nd edition AD&D, you subtracted the target's AC from your THAC0 and then tried to roll above the resulting number in order to hit. This meant that lower ACs were better, in contrast with most other stats. This inconsistency was obviously what made 2nd edition superior to all other editions with their regularized mechanics and simplified rules.