1) Ageless 2) Deathless 3) Enduring 4) Eternal 5) Everlasting 6) Immortal 7) Permanent 1) Deathless 2) Immortal 3) Never dying 4) Unending 
When a creature with undying is put into a graveyard from the battlefield, if it had no +1/+1 counters on it, it is returned to the battlefield under its owner's control with a +1/+1 counter on it. Undying first appeared in Dark Ascension. This keyword is very similar to persist.
• (a.) Not dying; imperishable; unending; immortal; as, the undying souls of men.
deathless; unending.