Isothermy (from the Greek words iso - “equal” and therme - “heat”) is a state of constant temperature of a substance.
An isothermal process is a process that occurs at a constant temperature. For example, isothermal expansion or compression of a gas means that the pressure of the gas changes but the temperature remains the same. In an isothermal process, heat transfer occurs in such a way that it compensates for the thermal effects of the process itself, maintaining a constant temperature.
Isothermal processes are widely used in physics and chemistry to study the behavior of substances at constant temperature. For example, adsorption isotherms show how much gas or vapor can be adsorbed on the surface of a solid at different pressures but the same temperature.
Thus, isothermy means maintaining a constant temperature of a substance during the process of physical or chemical transformations. This is an important condition for many experiments and processes.