Originally Posted by Satrina
But when I dodge I take no damage, so that really is mitigation!
As noted, that is technically true. The difference between mitigation and avoidance is a pedantic difference, but the big thing is that you can account for mitigation from armour in calculations since it is a constant. The damage you don't take because you dodged, parried or were missed, or because your block value was bigger than the hit, are all based on probability. You can't determine when that reduction is going to happen (and in the case of block you can't always know that your block value will be enough to block a whole hit). Because of that nature, you can't use it in calculations. That's why we separate them in definition because otherwise it gets terribly confusing when trying to show theorycraft, when trying to explain it all to new people - basically everywhere that matters.
Block is funny since it can act as mitigation when your block value is too low to absorb an entire hit, and it can be avoidance when it is. It's always based on probability, so it is not properly counted with your mitigation. These days it is very rarely a complete avoidance of damage, so it is not properly counted with your avoidance. Generally it is a third term when used in theorycrafting.
We generally add a third term to be the all-encompassing one: Mitigation for armour, Avoidance for dodge/parry/miss, and damage reduction when referring to the combination of them, including block.
There's a reason people come here for this kind of information, and not to whatever other place. We have consistent definitions like these because it helps us to describe stuff that can be fairly complex to people in as simple a way as possible.