Temperature

Temperature is a physical quantity that quantitatively expresses the perceptions of hot and cold. The temperature is measured with a thermometer.

Thermometers are calibrated in various temperature scales, which historically have relied on multiple 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), the latter being mainly used for scientific purposes. The kelvin is one of the seven base units of the International System of Units (SI).

Absolute zero, i.e. zero kelvin or −273.15°C, is the lowest point on the thermodynamic temperature scale. Experimentally, it can only be approached very closely but not achieved, as recognized by the third law of thermodynamics. It would be impossible to extract energy in the form of heat from a body at this temperature.

Temperature is important in all areas of natural science, including physics, chemistry, earth science, astronomy, medicine, biology, ecology, materials science, metallurgy, engineering mechanics, and geography, as well as most aspects of daily life.