Definition

Cryonics is a technique intended to hopefully save lives and greatly extend lifespan. It involves cooling legally-dead people to liquid nitrogen temperature where physical decay essentially stops, in the hope that future scientific procedures will someday revive them and restore them to youth and good health. A person held in such a state is said to be a "cryopreserved patient", because we do not regard the cryopreserved person as being inevitably "dead". 

However, this very simple idea is also very simple to misunderstand, because it involves several related but different scientific disciplines. For example, cryonics must be distinguished from cryogenics (the study of extreme low temperature on physical materials whether animate or inanimate). It must also be distinguished from cryobiology (the study of the effects of low temperature on biological material) because the study of low temperature effects on such material does not necessarily mean extreme low temperatures.

                                           

Sources: http://www.cryonicssociety.org/aboutcryonics_whatiscryonics.html

http://www.cryonics.org