What is the lowest possible temperature, at which molecular motion stops, called?

The lowest theoretically possible temperature is called absolute zero. It is defined as 0 kelvin, equal to −273.15°C or −459.67°F. At this limit, a system reaches its minimum possible thermal energy, which is why absolute zero forms the zero point of the Kelvin temperature scale.

The phrase “molecular motion stops” is a useful classroom shorthand, but quantum mechanics makes the full picture subtler. Particles retain zero-point motion and cannot simultaneously have perfectly known positions and momenta. Absolute zero therefore does not mean that every particle becomes perfectly motionless; it means that thermal excitations have been reduced to their lowest possible state.

Scientists can approach absolute zero using laser cooling, evaporative cooling, dilution refrigerators, and magnetic refrigeration, but the third law of thermodynamics says it cannot be reached through a finite physical process. Near this limit, matter can display superconductivity, superfluidity, and Bose–Einstein condensation. The triple point and freezing point are different concepts: they describe particular phase conditions, not the universal lower temperature limit.