Cryonics was a method of preserving individuals that could no longer be sustained by available medicine with the purpose of reviving them later on. The preservation was achieved by cooling to extremely low temperatures at which metabolic processes and decay were almost completely halted.
Cryonics was a fad in the late 20th century on Earth. A number of cryonic companies suffered a loss of reputation when power failures led to problems with their refrigeration systems. These scandals resulted in the stock of those companies losing much of their value. The fad did not continue much beyond the mid-21st century.
In 2364, the USS Enterprise-D intercepted a cryonics satellite in the Kazis binary system which had been launched into Earth orbit in 1994 but drifted out of orbit. Doctor Beverly Crusher was able to revive three of the Humans aboard who had not been subject to preservation failure: L.Q. Clemonds, Ralph Offenhouse, and Clare Raymond. (TNG: "The Neutral Zone")