Non Armor Defenses
Non-Armor Defenses, or NADs, is a /tg/ term derived from a game mechanic in Dungeons & Dragons, which became a part of the game with the 3rd edition.
Back in Advanced Dungeons & Dragons, players had a wide array of class-based (and sometimes race-modified) saving throws they could make against different kinds of threats; these got kind of convoluted, with separate saving throws for both spells cast directly by a caster and for spells cast from a wand, for example. So, when third edition rolled into town, they discarded the extensive list and replaced it with three new defenses; a player just had to roll the appropriate save when it was called for.
Fortitude covers how tough a character is, and thus how well they can resist things like poison or disease.
Will covers their strength of will, which defines how easily they can resist mental attacks.
4th edition changed these by switching them from being a save the player made into static defenses, which is arguably where the name came from; now, Fort, Ref and and Will were player scores the DM had to roll against, like Armor Class, rather than saves the player had to make.
5th edition either expanded on or abandoned NADs entirely, depending on how you look at it, replacing the whole system with a universal "make a saving throw with a bonus/penalty from the relevant ability score" style mechanic.