1.(Machinery) A gearing device which causes the drive shaft of an automobile or other machine to rotate at a speed greater than that of the engine crankshaft.
2.(Automotive) The gear setting of a motor vehicle at which the overdrive (1) is operative; as, cruising at 80 mph in overdrive.
3.Hence: (fig.) A state of unusually intense activity; as, when they got their new supercomputer, their research kicked into overdrive; one look at her made his heart jump into overdrive; used mostly of the activities of persons or groups.
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