Cryonics is a method of preserving individuals that can no longer be sustained by available medicine with the purpose of reviving them later on. The preservation is achieved by cooling to extremely low temperatures at which metabolic processes and decay are almost completely halted.
In 2364, USS Enterprise-D intercepted a cryonics satellite in the Kazis system which had been launched into Earth orbit in 1994 but drifted out of orbit. Dr. Crusher was able to revive three of the Human bodies aboard that had not been subject to preservation failure. (TNG: "The Neutral Zone")