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. They are also ignored by the wither.

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

Trivia

 * Because giants need to have a light level both less than 8 and greater than 11 to spawn, they don't spawn from monster spawners. However, this spawn condition can be bypassed with a custom spawner, if it attempts to spawn an entity with valid spawn conditions riding a giant.
 * If the player names a giant using a name tag, its name appears inside its head.
 * Changes made to the zombie model (for example, using a resource pack) change the giant's as well.
 * Notch mentioned Giant Zombies as a suggestion for a prefix mob along with other examples: "Burning Spider" and "Cobble Creeper" in a Bro-craft stream.
 * Notch removed the giants spawning capabilities because "they are too overpowered". He did not remove them entirely because "they are just too cool to remove".
 * If a giant is riding a minecart, it hovers way above it.
 * If a mob is riding a giant, unless it's a large slime or magma cube, a ghast, or the ender dragon, it is hidden inside the giant's head.
 * Players are unable to attack a giant's head and torso unless they are positioned near the bottom of the giant. Projectiles such as arrows can hit these areas as normal.
 * Giants, being capable of dealing to, and having  health, are capable of killing players without armor in one hit even in Easy difficulty.
 * Killing a Giant cannot give the player the Monster Hunter Advancement.
 * A giant chicken jockey causes the giant to "hover" above it.