Why is Armor Class called that way?

Question

I want to teach the game to some new player, and while doing homemade character sheets, for simplicity, I wanted to use the term "Defense" instead of "Armor class" (which I found a bit unintuitive).

Is there some reason at start to not just called it Armor Class?

I couldn't find any use of the word "Defense" or "Defence" alone in the game, and we already use the simple term attack, so why not?

PS: note that I'm French, perhaps the term is more commonly used in English language, that in French, which was translated literally as "Classe d'Armure"

Answer 1

Armor Class is a type of Difficulty Class

In Dungeons & Dragons 5e (and previous editions too if I'm not mistaken), usually the way to see if you are successful at doing something is to roll a d20, add relevant bonuses, and see if it matches the number you need to succeed. The number you need to succeed is called the Difficulty Class. This is true for skill checks, saving throws, and attack rolls.

The difficulty class you need to meet for attack rolls is called the Armor Class because usually, for player characters, the main way to determine this number is based on what armor they are wearing.

QuadraticWizard's answer has some great history but I think this common sense answer will help your new player understand D&D better.

Answer 2

It derives from the wargames which inspired D&D.

The 1971 Chainmail miniatures game which inspired D&D included a table for one-on-one combat, which determined hit or miss based on the attacker's weapon and defender's armor. This appears to be the origin of D&D's term "armor class", which referenced the Chainmail rules. The meaning appears to be similar to "classification", as in how to sort, rank, or group different things.

The term "class" is widely used throughout the Chainmail rules to refer to a category, group, ranking, classification, or type of things, including weapons, armor, cannons, heroes, wizards, elementals, and undead. "Class" is similarly used in the wargame Don't Give Up the Ship, where it is used to categorize different ships based on tonnage. This may have been a common naming convention in among wargamers in the 1970s.

The man-to-man combat rules in Chainmail provide a table to determine the roll required to hit, which takes into account weapon type vs armor type. Some armor types share a column in the melee table; e.g. leather and padded armor are functionally the same. The ranged weapon table refers to "Class of armor worn by the defender", giving a number to each of eight armor groups.

The original 1974 Dungeons & Dragons provides an alternative combat system which closely matches the Chainmail table, except that the number is now referred to specifically as "Armor Class", and it is higher level rather than weapon type which determines the number which must be rolled to score a hit.

Originally, then, "armor class" simply meant "type of armor worn", with the implication that some armor is better than others. Since most monsters do not wear armor, and there are other magical means of increasing armor class in D&D, the term "armor class" soon became decoupled from the actual armor types, and referred only to the to-hit number. However, it was retained largely out of D&D tradition, which has always kept a lot of archaic words and ideas as part of its flavor.