in thermodynamics, change occurring within a system as a result of transfer of energy to or from the system in the form of work only. A rapid expansion or contraction of a gas is very nearly adiabatic. Any process that occurs within a container that is a good thermal insulator is also adiabatic. Adiabatic processes are characterized by an increase in entropy, or degree of disorder, if they are irreversible and by no change in entropy if they are reversible. Adiabatic processes cannot decrease entropy.