Dies and died replace the phrase "is put into a graveyard from the battlefield" or "has been put into a graveyard from the battlefield". In contrast to the similar destroy, these are not keyword actions. Their main function is to be used as a condition for triggered abilities. As such it mirrors enters the battlefield effects.
Creatures die if they take lethal damage or have their toughness reduced in less than 1, as a State-based action. Planeswalker die if they have no loyalty counters on them. Both of them die if they get destroyed, or any other effect or spell that make them go from the battlefield to the graveyard, such as Sacrifice.
A creature or planeswalker does not die if a replacement effect sends it to another zone (usually Exile), similar to tokens that are put into a zone other than the graveyard. They cease to exist, but abilities that check if a creature or planeswalker dies do not trigger.
"Dies" used to be a slang term for this occurrence before and was reintroduced in Magic 2012, in anticipation of Innistrad and the Morbid mechanic. It used to be exclusive to creatures while other card types still use the old phrasing. As of Core Set 2019, a planeswalker is also considered to "die" when sent from the battlefield to the graveyard (Ajani’s Last Stand).
- A creature or planeswalker “dies” if it is put into a graveyard from the battlefield. See rule 700.4.
- 700.4. The term dies means “is put into a graveyard from the battlefield.”
- Mark Rosewater (October 10, 2018). "If a color can have and ETB on a creature can if to the same effect on a death trigger?". Blogatog. Tumblr.