THAC0

An acronym from the tabletop role playing game Dungeons & Dragons. It represents "To Hit Armor Class 0".

It is typically a number between 1 and 20. When a character is attacking, a 20 sided die is rolled. If the number on the die and the defender's armor class add up to a number equal to or less than the THAC0 of the attacker, the attack is successful.

In the current D&D Rules, THAC0 is no longer used.