The Collaborative International Dictionary of English v.0.48:
gearbox \gear"box`\ n. the metal casing in which a train of gears is sealed. Syn: gear case. [WordNet 1.5]WordNet (r) 3.0 (2006):
gearbox n 1: the shell (metal casing) in which a train of gears is sealed [syn: gearbox, gear box, gear case]