n. 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. [ PJC ]
2. (Automotive) The gear setting of a motor vehicle at which the overdrive{ 1 } is operative; as, cruising at 80 mph in overdrive. [ PJC ]
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. [ PJC ]