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Temperature scales differ in two ways: the point chosen as zero degrees and the magnitudes of the incremental unit of temperature. ( January 2021) ( Learn how and when to remove this template message) Unsourced material may be challenged and removed. Please help improve this article by adding citations to reliable sources. This section needs additional citations for verification.

9.5 Generalized temperature from single-particle statistics.

9.4 Definition from statistical mechanics.

6.4 Thermodynamic equilibrium axiomatics.

6.2 Bodies in a steady state but not in thermodynamic equilibrium.

6.1 Bodies in thermodynamic equilibrium.

3.2.1 Microscopic statistical mechanical scale.

2.5 Statistical mechanical versus thermodynamic temperature scales.

Temperature is important in all fields of natural science, including physics, chemistry, Earth science, astronomy, medicine, biology, ecology, material science, metallurgy, mechanical engineering and geography as well as most aspects of daily life. The SI temperature is not defined for such components, because they are not moving freely. With carefully regulated small flows of energy, a body can be held in a state that is practically steady though not of thermodynamic equilibrium, in which microscopic components of the body can be regarded as virtually having mathematically defined negative absolute thermodynamic temperature, but such a body is hotter than a body imagined to be at absolute zero temperature. Every actual physical body in a state of thermodynamic equilibrium has a positive absolute temperature, both SI and thermodynamic. A body can have a strictly physically defined thermodynamic temperature only when it is in a state of thermodynamic equilibrium. It would be impossible to extract energy as heat from a body at that temperature. Experimentally, it can be approached only very closely, but not actually reached, as recognized in the third law of thermodynamics.

The coldest that a body can be imagined to be is when it has a temperature of absolute zero. The most common scales are the Celsius scale (formerly called "centigrade", with the unit ☌), the Fahrenheit scale (with the unit ☏), and the Kelvin scale (with the unit K), the latter being used predominantly for scientific purposes and is the primary temperature scale defined by the International System of Units (SI). It is calibrated in various temperature scales that historically have relied on various reference points and thermometric substances for definition. Temperature is measured with a thermometer. There are three types of temperature scale: those, such as the SI scale, that are defined in terms of the average translational kinetic energy per freely moving microscopic particle, such as atom, molecule, or electron, in a body those that rely solely on purely macroscopic properties and thermodynamic principles, such as Kelvin's original definition and those that are not defined by theoretical principles, but are defined by convenient empirical properties of particular materials.

Temperature is a physical quantity that expresses the hotness of matter or radiation.