Absolute Zero

Absolute zero refers to the lowest theoretically possible temperature, at which the motion of particles that constitute matter would be minimal.
It is defined as being 0 Kelvin (K) or -273.15 degrees Celsius.
At absolute zero, substances have minimum internal energy.
No process can lower the temperature of any system to absolute zero. This is referred to as the Third Law of Thermodynamics.
Reaching absolute zero would require an infinite number of steps, thus is practically impossible.
The concept of absolute zero is crucial to understanding thermal physics and thermodynamics.
All theoretical models and laws of physics including quantum mechanics and kinetic theory of gases are based on the concept of absolute zero.
The Kelvin scale of temperature starts from zero, which corresponds to absolute zero, thus there are no negative temperatures in the Kelvin scale.

Practical Implications

At or near absolute zero, many common materials such as metals exhibit superconductivity, and gases become superfluids.
Superconductivity is a phenomenon of exactly zero electrical resistance and rejection of magnetic flux fields occurring in certain materials when cooled below a characteristic critical temperature.
Superfluidity is a state of matter in which matter behaves like a fluid with zero viscosity.
The quest to achieve temperatures close to absolute zero led to the development of techniques such as laser cooling and evaporative cooling which are used in cold atom laboratories.
The most precise timekeeping devices known to man, atomic clocks, also use atoms cooled to near absolute zero.