[RPG] Why is Armor Class called that way

dnd-5egame-designhistory-of-gamingterminology

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"

Best Answer

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.