Cheating Death is the rare ability of not dying without being immortal, no matter how impossible it seems to survive the killing-event.
The ability to cheat death is used by authors who reveal that the one you think was dead is, currently, alive and well. Therefore, it is used to bring back to life a character without actually killing the character.
This ability is NOT to be confused with Immortality or Resurrection. While Immortality implies that a character can never die, and outright resurrection would indicate that the character was dead but is alive again, a character who cheats death never actually dies. Rather, though they seemed to be in a situation that was impossible to survive, they managed to cheat those odds.
The most notable example of this would be the The Joker, who is not immortal but has cheated death many times.