Armor Class

The simplest, and possibly least useful, system of simulating the protection of armor and dodging, Armor Class is what you have to roll against to score a hit in Dungeons and Dragons.

Usually, the way it works is you have a Base AC, with a Dexterity bonus when naked. Put on some armor, and your Base AC improves (whichever way that is in your edition), but your maximum Dex bonus goes down as well, with better armor offering better Base AC.

Early editions of D&D used "THAC0" and a negative AC system; that is, lower AC was better since everything subtracted from it, and AC could go below 0. Starting with 3rd Edition, AC became a matter of "You have a to hit bonus; add that to a d20, and if you go over their AC, you hit.", which had the advantage of vastly simplifying the math required to the point that only a very few games that are intentionally trying to be "old fashioned", such as NetHack, use the old "lower is better and negative numbers" style of Armor Class.

Some games like Dungeons & Dragons 4th Edition introduced a variant to AC called Non Armor Defenses, which are like AC, but had other stats contribute to it and tended to not have any influence from armor. These basically allowed players to determine more accurately what sort of weakness an enemy had (e.g. a powerful wizard with high Will and good AC/Reflex, but a piss-poor Fortitude).