Temporary Hit Points
A variety of sources can grant temporary hit points—small reservoirs of stamina that insulate a creature from losing actual hit points.
Not Real Hit Points: Temporary hit points aren’t healing, but rather a layer of “insulation” that attacks have to get through before they start dealing real damage to a target. Don’t add temporary hit points to a creature’s current hit points (if an adventurer has 0 hit points or fewer, he or she still has 0 or fewer after receiving temporary hit points). Keep track of them as a separate pool of hit points.
Don’t Count toward Maximum: Temporary hit points don’t count when comparing a creature’s current hit points to its maximum hit points, when determining whether the creature is bloodied, or for other effects that depend on its current hit points.
Lose Temporary Hit Points First: When a creature takes damage, first subtract its temporary hit points from the damage. Any remaining damage reduces its current hit points.
Not Cumulative: If a creature receives temporary hit points multiple times, use the highest value as its temporary hit point total. Do not add the values together. For example, a creature receives 5 temporary hit points and later receives 5 temporary hit points again before the first 5 were used. It now has 5 temporary hit points, not 10.
Likewise, if a creature’s temporary hit points have been reduced and the creature receives temporary hit points again, it benefits from the higher number. For example, a creature gains 10 temporary hit points and takes 8 damage, reducing its temporary hit points to 2, then receives 5 temporary hit points. It now has 5 temporary hit points, not 7.
Until a Rest: A creature’s temporary hit points last until they’re reduced to 0 or until the creature takes a short rest or an extended rest.
Published in Player's Handbook, page(s) 293, Rules Compendium, page(s) 258.