Hit Points

Over the course of a battle, adventurers and monsters take damage from attacks. Hit points measure the ability of a creature to stand up to punishment, turn deadly strikes into glancing blows, and stay on its feet throughout a battle. Hit points represent more than physical endurance. They also represent skill, luck, and resolve—all the factors that combine to help a creature stay alive in combat.
    An adventurer’s maximum hit points are determined during character creation and change as the character advances in level. A monster’s hit points depend on its level and role and are noted in its stat block, along with its bloodied value.
    Whenever a creature takes damage, subtract that number from its current hit points. As long as that current hit point total is higher than 0, the creature can keep on fighting. A monster normally dies at 0 hit points; an adventurer whose current hit point total drops to 0 or lower is dying.
    Powers, abilities, and actions that restore hit points are forms of healing. Characters (and some monsters) might regain hit points through rest, heroic resolve, or magic. When a creature heals, add the number of hit points regained to its current hit points. A creature can heal up to its maximum hit point total but can’t exceed it.

Hit Points
Damage reduces hit points.

Maximum Hit Points: A character’s class, level, and Constitution score determine his or her maximum hit points. A monster’s maximum hit points are noted in its stat block. A creature’s current hit points can’t exceed this number.

Bloodied Value: The bloodied value of a typical creature equals one-half its maximum hit points. A creature is bloodied when its current hit points drop to its bloodied value or lower. Certain powers and effects work only against a bloodied target or work better against such a target. An object is never considered to be bloodied.
    When an unbloodied creature is killed outright, the creature does not become bloodied in the process. It is just dead. This rule means a minion, which has only 1 hit point, is bloodied only if an attacker reduces the minion to 0 hit points but knocks it unconscious (see “Knocking Creatures Unconscious") instead of killing it.

Dying: When a character’s current hit points drop to 0 or lower, the character is dying.

Published in Player's Handbook, page(s) 293, Rules Compendium, page(s) 256.