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"