In physics and engineering, cryogenics is the study of very low temperature (below −150 °C, −238 °F or 123 K), how those temperatures are reached, and what materials do at those temperatures on a quantum, atomic, and molecular level.
Scientists who study cryogenics use the Kelvin scale as it is more precise. 0 on the Kelvin scale is known as absolute zero.
Recently it was discovered that the temperature scale is in fact a loop: a gas was chilled to a temperature below absolute zero and the particles within that gas began behaving erratically, as if they were infinitely hot. This has lead to speculation as to the nature of dark energy and is nevertheless quite strange.
There are a number of different applications of cryogenics within science.
Cryobiology is the study and practice of placing organisms very close to a state of cryopreservation.
Cryobiology is distinct from cryonics, in which organisms are fully frozen.
Cryosurgery is when only a specific part of an organism is frozen so as to remove a tumor or wart.
Cryoelectronics is a field of research involving the effects of extremely low temperatures upon electricity. These topics include topological insulators, the Quantum Hall effect, 2-dimensional gases, and superconductivity—when a substance is cooled below a critical temperature so that exactly zero electrical resistance occurs and all magnetic fields are expulsed.
Cryotronics is the practical application of cryoeletronics.
Cryogenics may also be used as source of fuel and has applications within art and culinary science.
Thanks to Oneiromancer for help elaborating cryolectronics.