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.

Trivia

 * Despite Giants being over-sized variant of the Zombie, it's not considered a undead mob.
 * Zombie Horses it's, so far, the only undead mob completely passive.