Temperature is a physical quantity that expresses the subjective perceptions of hot and cold. Temperature is measured with a thermometer, historically calibrated in various temperature scales and units of measurement. The most commonly used scales are the Celsius scale, denoted in °C (informally, degrees centigrade), the Fahrenheit scale (°F), and the Kelvin scale.
The kelvin (K) is the unit of temperature in the International System of Units (SI), in which temperature is one of the seven fundamental base units. The lowest theoretical temperature is absolute zero, at which the thermal motion of all fundamental particles in matter reaches a minimum. Although classically described as motionless, particles still possess a finite zero-point energy in the quantum mechanical description. Absolute zero is denoted as 0 K on the Kelvin scale, −273.15 °C on the Celsius scale, and −459.67 °F on the Fahrenheit scale.
Based on the historical development of the kinetic theory of gases, a simplified description of fluid matter, temperature is proportional to the average kinetic energy of the random microscopic motions of the constituent microscopic particles, such as electrons, atoms, and molecules, but rigorous descriptions must include all quantum states of matter. Temperature is important in all fields of natural science including physics, geology, chemistry, atmospheric sciences, medicine and biology as well as most aspects of daily life.