In classical thermodynamics and kinetic theory, temperature reflects the average kinetic energy of the particles in a system, providing a quantitative measure of how energy is distributed among microscopic degrees of freedom. Temperature reflects the average kinetic energy of the vibrating and colliding atoms making up a substance. It is a numerical expression of the everyday experience of hotness or coldness.

Temperature
Thermal vibration of a segment of a protein's alpha helix. Its amplitude increases with temperature.
Common symbols	T
SI unit	K
Other units	°C, °F, °R, °Rø, °Ré, °N, °D, °L, °W, Planck units
Intensive?	Yes
Derivations from other quantities	${\displaystyle {\frac {pV}{nR}}}$, ${\displaystyle {\frac {dq_{\text{rev}}}{dS}}}$
Dimension	${\displaystyle {\mathsf {\Theta }}}$

Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied on various reference points and thermometric substances for definition. The most common scales are the Celsius scale with the unit symbol °C (formerly called centigrade), the Fahrenheit scale (°F), and the Kelvin scale (K), with the third being used predominantly for scientific purposes. The kelvin is one of the seven base units in the International System of Units (SI).

Absolute zero, i.e., zero kelvin, 0 K = −273.15 °C, is the lowest point in the thermodynamic temperature scale. Experimentally, it can be approached very closely but not actually reached, as recognized in the third law of thermodynamics. It would be impossible to extract energy as heat from a body at that temperature.

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.