What is the term for a process that occurs at constant temperature?

What is the term for a process that occurs at constant temperature? The answer is isothermal. An isothermal process maintains a constant temperature, so ΔT = 0. The word combines Greek roots meaning “equal” and “heat.” Constant temperature does not mean that pressure, volume, or heat transfer must also remain constant.

For an ideal gas, an isothermal change follows Boyle’s law: pressure multiplied by volume remains constant when the amount of gas and temperature are fixed. Expansion lowers the pressure as volume increases, while compression raises the pressure as volume decreases. On a pressure–volume diagram, these paths are called isotherms.

In an ideal gas, internal energy depends only on temperature, so its internal energy does not change during an isothermal process. During expansion, heat must enter the gas to replace the energy leaving as work; during compression, heat generally leaves the gas. This differs from an adiabatic process, where no heat is transferred. Isochoric and isobaric processes instead hold volume and pressure constant.