Adiabatic, from the classic Greek ἡ διαβασις (the going through) and ἀ (negation, non-), is an adjective referring to a wall of a container that is completely impermeable by heat. No heat can go through an adiabatic wall. From this usage is derived adiabatic process that in thermodynamics refers to a process that takes place without absorption or generation of any heat.
In mechanics, and especially quantum mechanics, adiabatic can also refer to a time-dependent process, where adiabatic means "infinitesimally slow". An adiabatic change takes an amount of time that goes to infinity, that is, the change takes so much longer than other changes of interest that its duration may be taken to be infinite.