Temperature
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Temperature is a physical property of a system that underlies the common notions of hot and cold; something that is hotter generally has the greater temperature. Temperature is one of the principal parameters of thermodynamics. On the microscopic scale, temperature is defined as simply the average energy of microscopic motions of a single particle in the system per degree of freedom. On the bulk scale, common to non-scientists, temperature is defined as that unique physical property that is shared between two otherwise entirely unlike things that happen to be in thermal equilibrium with each other (meaning, no net heat energy is exchanged between them). For a solid, these microscopic motions are principally the vibrations of the constituent atoms about their sites in the solid. For an ideal monatomic gas, the microscopic motions are the translational motions of the constituent gas particles. For multiatomic gas vibrational and rotational motion should be included too.
Celsius is, or relates to, the Celsius temperature scale (previously known as the centigrade scale). The degree Celsius (symbol: °C) can refer to a specific temperature on the Celsius scale as well as serve as unit increment to indicate a temperature interval (a difference between two temperatures or an uncertainty). “Celsius” is named after the Swedish astronomer Anders Celsius (1701 – 1744), who developed a similar temperature scale two years before his death.
Fahrenheit is a temperature scale named after the German-Dutch physicist Daniel Gabriel Fahrenheit (1686–1736), who proposed it in 1724.
In this scale, the freezing point of water is 32 degrees Fahrenheit (written "32 °F"), and the boiling point is 212 degrees, placing the boiling and freezing points of water exactly 180 degrees apart. On the Celsius scale, the freezing and boiling points of water are exactly 100 degrees apart, thus the unit of this scale, a degree Fahrenheit, is 5?9 of a degree Celsius. The Fahrenheit scale coincides with the Celsius scale at -40 °F, which is the same temperature as -40 °C.
The Kelvin (symbol: K) is a unit increment of temperature and is one of the seven SI base units. The Kelvin scale is a thermodynamic (absolute) temperature scale where absolute zero — the coldest possible temperature — is zero kelvins (0 K).
The Kelvin scale and the kelvin are named after the Northern Irish physicist and engineer William Thomson, 1st Baron Kelvin (1824 – 1907), who wrote of the need for an “absolute thermometric scale.”