Isothermal Process
Definition
A process that happens at constant temperature — the system stays in thermal balance with its surroundings the whole time.
Technical definition
A thermodynamic process with ΔT = 0. For an ideal gas, internal energy depends only on temperature, so all heat added converts entirely to work (Q = W), and pressure times volume stays constant — Boyle's law.
Example
A balloon squeezed very slowly stays at room temperature because heat has time to flow in and out — nearly isothermal. Squeeze it fast and it warms up instead, because the compression outruns heat exchange.
Isothermal is not adiabatic. Isothermal means temperature stays constant (heat flows freely to compensate); adiabatic means no heat flows at all (temperature changes instead). They are opposites in a sense: slow compression in a water bath is isothermal, while the fast compression in a diesel engine is nearly adiabatic — and the temperature outcomes are completely different.