Temperature

Temperature is a physical quantity that characterizes thermal state and determines the direction of spontaneous heat transfer between systems in thermal contact.

Thermal equilibrium: A state in which systems in thermal contact have no net heat transfer between them. Equal temperatures mark the absence of net heat flow.
Heat: Energy transferred between systems because of a temperature difference. Heat is energy in transit, whereas temperature characterizes a system's thermal state.
Thermometer: An instrument that measures temperature through a calibrated physical response to thermal state. Thermometers infer temperature from properties that vary predictably with it.
Negative temperature: A thermodynamic temperature below zero on the Kelvin scale in systems with an upper bound on energy. It is hotter than any positive temperature, despite its negative numerical value.
Zeroth law of thermodynamics: The thermodynamic law stating that systems each in thermal equilibrium with a third system are in equilibrium with each other. It establishes temperature as a property that can be compared consistently.
Internal energy: The total microscopic kinetic and potential energy contained within a system. Temperature relates to internal energy, but the two quantities are not interchangeable.
Celsius: A temperature scale that assigns 0 °C to water's freezing point and 100 °C to its boiling point at standard pressure. It is a widely used scale for everyday and scientific measurements.
Effective temperature: A temperature-like parameter used to describe a system that is not in full thermal equilibrium. It can summarize selected behavior without defining a single equilibrium temperature.
Statistical mechanics: The framework that connects macroscopic properties of matter to the statistical behavior of microscopic constituents. It derives temperature from how energy is distributed among microscopic states.
Entropy: A thermodynamic state function that quantifies the dispersal of energy and the number of accessible microscopic arrangements. Temperature is related to how entropy changes as a system's energy changes.

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