Tonguez
A suffusion of yellow
once you have a definition of “what is Life” you dont need to define “what is undead” as the creature type description already tells us that Undead are once-living creatures. Its not a different state just a yes/no switch.But as I say, "what is life" is irreverent to the question of "what is undead". Lots have things have been suggested, but they have all been dismissed on the basis of being "boring". If you can't define what it means to be undead, you cannot say how it differs from life.
We are further told that the “horrifying state of undeath is brought about through the practice of necromantic magic or some unholy curse*.”
We also get told there re two categories for undead being walking corpses, and bodiless spirits.
*we know too that in DnD Unholy Power is an objective thing that can affect objects (Unholy Weapons) and creatures, regardless if individuals object to the connotations of such Holy/Unholy powers existing.