Description: Absolute zero is the zero point of the kelvin temperature scale, corresponding to -273.15 degrees on the Celsius scale and -459.67 degrees on the Fahrenheit scale. This choice of zero point is motivated by fundamental physics: for a classical system, the temperature of absolute zero would correspond to a state where all of the particles are at perfect rest, each with kinetic energy zero. In the real world, basic effects of quantum theory mean that this state of complete rest will never be reached.

In the language of thermodynamics, which describes general systems and their abilities to exchange heat and other forms of energy, an idealized system at temperature absolute zero would be a system from which no heat could be extracted at all. In practice, it is impossible to bring a system into that ideal state. This is codified by the so-called 3rd law of thermodynamics, also called Nernst's theorem: we can get arbitrarily close to absolute zero, but we can never reach it.

Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".