The practice of using Sorcery to restore a dead person to life.
In the days before the Interregnum, many healers made use of sorcery to save the very nearly dead. Some wizards were likely powerful enough to save lives that under normal circumstances might have been considered already dead.
Revivification is only possible when the body is less than 3 days dead, the brain is undamaged, and the individual was not killed by a Morganti blade.
Sorcerers usually charge anywhere from 3,000 to 5,000 Imperials for this service, so unless you have a lot of money (or friends like Aliera e'Kieron or The Necromancer), you should try not to get killed too often.
It is possible for a sorcerer (especially a member of the Left Hand of the Jhereg) to place a spell upon a body that will prevent revivification. If the sorcerer is skillful enough, this spell can only be removed by the one who placed it.