This article relies largely or entirely on a single source. (April 2013)
In some role-playing games, armor class (abbreviated AC) is a derived statistic that indicates how difficult it is to land a successful blow on a character with an attack. It represents either a character's protective equipment, ability to dodge attacks, or a combination of the two.
In earlier versions of Dungeons & Dragons, a lower AC indicated that a creature was more difficult to hit. In Advanced Dungeons & Dragons, an unarmored human had an AC of 10, and armor lowered a character's armor class. Powerful creatures would usually have an armor class lower than 0.
In Advanced Dungeons & Dragons 2nd Edition, a character or monster's ability to strike successfully was indicated by its THAC0, the minimum roll needed on a 20-sided die "To Hit Armor Class 0". The die roll needed to hit other armor classes could be computed by subtracting the armor class from the THAC0. The lower one's THAC0, the more likely a hit would be successful. This system replaced combat tables in the 2nd edition of AD&D, but was officially abandoned in the 3rd edition of D&D (2000).
In third edition D&D, the armor class system was effectively reversed. An unarmored human still had an AC of 10, but wearing additional armor and/or wielding a shield increased armor class. Thus, a creature with an AC of 0 in second edition would have an armor class of 20 in third edition, and vice versa. This system persists into the fifth edition rules. A wide range of factors affects armor class in both systems, including a character's dexterity, use of various combat techniques (such as forms of parrying), and the quality and material composition of the armor worn.
The concepts of armor class and hit points originated in a set of rules for a naval battle game set during the American Civil War. Dungeons & Dragons co-creator Dave Arneson adopted the concepts in miniatures games that he ran shortly before the first edition of D&D was written.
Numerous Dungeons & Dragons–derived tabletop role-playing and role-playing video games use a similar armor class system. Many games with unrelated systems use the term to represent a character's ability to avoid damage or reduce damage taken.