An
isothermal process is a
change of a
system, in which the
temperature remains constant: Δ
T = 0. This typically occurs when a system is in contact with an outside thermal reservoir (
heat bath), and the change occurs slowly enough to allow the system to continually adjust to the temperature of the reservoir through
heat exchange. In contrast, an
adiabatic process is where a system exchanges no heat with its surroundings (
Q = 0). In other words, in an isothermal process, the value Δ
T = 0 and therefore Δ
U = 0 (only for an ideal gas) but Q ≠ 0, while in an adiabatic process, Δ
T ≠ 0 but Q = 0.