In thermodynamics, an isothermal process is a type of thermodynamic process in which the temperature T of a system remains constant: ΔT = 0. This typically occurs when a system is in contact with an outside thermal reservoir, and a change in the system occurs slowly enough to allow the system to be continuously adjusted to the temperature of the reservoir through heat exchange (see quasi-equilibrium). In contrast, an adiabatic process is where a system exchanges no heat with its surroundings (Q = 0).

Simply, we can say that in an isothermal process

• ${\displaystyle T={\text{constant}}}$
• ${\displaystyle \Delta T=0}$
• ${\displaystyle dT=0}$
• For ideal gases only, internal energy ${\displaystyle \Delta U=0}$

while in adiabatic processes:

• ${\displaystyle Q=0.}$