ZERO, noun Cipher; nothing. The point of a thermometer from which it is graduated. zero in the thermometers of Celsius and Reaumur, is at the point at which water congeals. The zero of Fahrenheits thermometer is fixed at the point at which the mercury stands when immersed in a mixture of snow and common salt. In Wedgewoods pyrometer, the zero corresponds with 1077 degrees on Fahrenheits scale.