A convenient operational definition of temperature is that it is a measure of the average translational kinetic energy associated with the disordered microscopic motion of atoms and molecules.
Temperature is expressed in degrees. There are two units of temperature in common use - Celsius (°C) and Fahrenheit (°F). Both make reference to the freezing and boiling points of pure water at normal atmospheric pressure.
Celsius (sometimes known as centigrade in the past) sets the freezing point at 0 degrees (0 °C) and the boiling point at 100 degrees (100 °C).
Fahrenheit sets the freezing point at 32 degrees (32 °F) and the boiling point at 212 degrees (212 °F).
Conversion of Fahrenheit to Celsius: °C = ( °F - 32 ) x 5 ÷ 9
Conversion of Celsius to Fahrenheit: °F = ( °C x 9 ÷ 5 ) + 32
Universally Celsius is the predominant system but Fahrenheit still holds sway in the USA.
An alternative scale, the Kelvin scale (after the famous Irish-born physicist), is used in scientific and engineering circles. It is a thermodynamic (absolute) temperature scale: it sets 0 K at the absolute zero: the coldest possible temperature in the universe, where an object has no thermal energy. The unit of the Kelvin scale (K) is equal to the unit of the Celsius scale (°C). Thus the freezing point of water at normal atmospheric pressure occurs at 273.15 K.
The Kelvin is one of the fundamental seven SI units. Today the Kelvin scale is defined by two points: the absolute zero and the triple point of water (273.16 K). No degree symbol (°) is used with the Kelvin symbol (K).
In US engineering fields another scale, the Rankine scale, is still used. As the Kelvin it is a thermodynamic (absolute) temperature scale, but one Rankine unit (°R) equals one Fahrenheit unit (°F). Thus, the freezing point of water at normal atmospheric pressure occurs at 459.67 °R.