Undead

Undead, by definition, refers to an entity that is dead, but still animate.

Classification
Within Minecraft, it refers to an in-game category of mobs that are generally hostile, damaged by potions of healing, healed by potions of harming, and are immune to drowning (except for husks and zombies, which turn into zombies and drowned if drowning, respectively) and poison damage.

Undead mobs also take extra damage from swords or axes enchanted with Smite.

Trivia

 * Despite the Giant being an oversized variant of the Zombie, it's not considered an undead mob.
 * Zombie Horses are, so far, the only completely passive undead mob.
 * For Ghasts and vexes, they are not undead mob because they are immune to Potion of healing damage.
 * Withers do not attack undead mobs but do attack every other mob.