Absolute zero is a theoretical temperature that is thought to be the lowest possible temperature.
absolute zero in British English
noun
the lowest temperature theoretically attainable, at which the particles constituting matter would be in the lowest energy states available; the zero of thermodynamic temperature; zero on the International Practical Scale of Temperature: equivalent to –273.15°C or –459.67°F
absolute zero in American English
the temperature at which matter has no heat and its molecules are completely still; theoretically, the lowest possible temperature: equal to about -273.16°C or about -459.69°F or 0°K