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 share the following characteristics:
 * Are damaged by and healed by ;
 * Are not affected by and ;
 * Take extra damage from Smite;
 * Do not take drowning damage (except for husks and zombies, which turn into zombies and drowned, respectively);
 * Are not targeted by the.

Some (but not all) undead mobs also possess these characteristics:
 * Are hostile to the player;
 * Catch fire under sunlight.

Trivia

 * Despite the giant being an oversized variant of the zombie, it's not considered an undead mob.
 * Despite the ghast's ghost-like appearance, it is not considered an undead mob.