Absolute zero is the point at which no further heat can be removed from an object. In classical terms, at zero temperature all molecules are standing still, there is no translation, rotation or vibration. In quantum mechanical terms all possible motions are in their ground state.
Commonly used temperature scales, such as Celsius and Fahrenheit, are based around everyday experience and thus have zero points vastly above this theoretical lowest possible temperature. Absolute zero corresponds to −273.15 °C on the Celsius temperature scale and to −459.67 °F on the Fahrenheit scale. It is expressed as 0° in specialist scales such as the Kelvin and the Rankine scales. The Kelvin scale is identical to the Celsius scale shifted downward by 273.15. That is, a temperature difference of 1 K is by definition identical to a temperature difference of 1 °C. Similarly, the Rankine scale is identical to the Fahrenheit scale shifted downward by 459.67 and a temperature difference of 1 °R is by definition identical to a temperature difference of 1 °F.
The third law of thermodynamics asserts that it is impossible to reduce the temperature of any system to exactly zero temperature. However, it is possible in the laboratory to reduce the speed of gas molecules to such an extent that their kinetic energy corresponds to a temperature of less than a nanokelvin (10−9 K) above the absolute zero. Although the absolute zero is theoretically unattainable, a nanokelvin temperature is very close to it.