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 practice, a process may be considered adiabatic if it occurs over a time interval short enough that no significant amount of heat is transferred into or out of the region of interest, regardless of the insulation properties of any surrounding container. Examples are the compression and power strokes in an internal combustion engine.
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.