What is the lowest possible temperature, 0 K, called?

The lowest possible temperature, 0 K, is called absolute zero. It equals −273.15 °C or −459.67 °F and represents the lower limit of thermodynamic temperature.

At absolute zero, a system has its minimum possible thermal energy under the relevant physical conditions. This does not mean every particle stops moving completely: quantum mechanics permits zero-point motion even in the lowest-energy state. For a perfect crystal, the third law of thermodynamics predicts that entropy approaches zero as the temperature approaches 0 K.

The idea developed from gas behavior and the search for an absolute temperature scale. In 1848, William Thomson, later known as Lord Kelvin, proposed a scale whose zero was independent of the properties of any particular substance. The kelvin scale now places absolute zero at exactly 0 K.

Absolute zero cannot be reached by a finite physical process, according to the unattainability principle of the third law, although laboratories can approach it extraordinarily closely using laser cooling, dilution refrigerators, and adiabatic demagnetization. “Freezing point” is not correct: water freezes at 0 °C, or 273.15 K, far above absolute zero.