(2) Absolute zero, temperature at which a thermodynamic system has the lowest energy. It corresponds to“273.15 °C on the Celsius temperature scale and to“459.67 °F on the Fahrenheit temperature scale. The notion that there is an ultimately lowest temperature was suggested by the behaviour of gases at low pressures: it was noted that gases seem to contract indefinitely as temperature decreases. It appeared that an“ideal gas” at constant pressure would reach zero volume at what is now called the absolute zero of temperature.