Thermal equilibrium

Thermal equilibrium is a state in which systems in thermal contact have the same temperature and exchange no net heat. Their macroscopic thermal properties remain stable over time.

Zeroth law of thermodynamics: The thermodynamic law stating that systems each in thermal equilibrium with a third system are in thermal equilibrium with each other. It makes temperature equality a consistent criterion for thermal equilibrium.
Thermodynamic system: A specified quantity of matter or region of space chosen for thermodynamic analysis. Equilibrium describes the state of one system or several systems considered together.
Thermal nonequilibrium: A condition in which temperatures or thermal properties vary in a way that can drive net heat transfer. A temperature difference marks the absence of thermal equilibrium and drives heat flow.
Thermometer: An instrument that measures temperature through a physical property that varies predictably with thermal state. A thermometer measures a system's temperature after reaching thermal equilibrium with it.
Temperature: A physical quantity that characterizes thermal state and determines the direction of spontaneous heat transfer. Systems reach thermal equilibrium when their temperatures become equal.
Thermodynamic state: A description of a system using macroscopic variables such as temperature, pressure, and volume. Thermal equilibrium is a particular condition of a system's thermodynamic state.
Thermodynamic equilibrium: A state in which a system has no macroscopic driving forces for change and is in thermal, mechanical, and chemical equilibrium. Thermal equilibrium is one component of the broader condition of thermodynamic equilibrium.
Calorimetry: The measurement of heat transferred during physical or chemical processes. Calorimetric measurements use equilibrium temperatures to infer energy exchange.
Heat: Energy transferred between systems because of a temperature difference. Thermal equilibrium means there is no net transfer of heat between the systems.
Thermal reservoir: A system with sufficiently large heat capacity to exchange energy while undergoing negligible temperature change. A reservoir can bring a smaller system to its own temperature with little change to itself.

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