Originally Posted by
Hemlock
Examples off the top of my head of the incorrectness of your "PC does"/"something affected PC" definition: being grappled is the result of someone else assaulting you, and yet it is resisted with a Strength (Athletics) or Dexterity (Acrobatics) check, not a save. Being grabbed by a mage's Telekinesis is an external assault, and yet it is resisted with a Strength check. Thirst is... well, it's not an external assault, yet according to at least part of the PHB it's resisted with a Con save, not a check. Otto's Irresistible Dance has no initial save, but when you try to stop it, you use your action to make a Wisdom save, not a Wisdom check. (Contrast this with e.g. Wrathful Smite, where you use your action to make a Wisdom check, not a Wisdom save.)