Transmission

Noun

1. The act of transmitting, e.g. data (signals) or electric power.
2. The fact of being transmitted.
3. Something that is transmitted, such as a message, picture, or a disease; the sending of such a thing.
4. The passage of a nerve impulse across synapses.
5. A device, usually an assembly of gears (gearbox) plus a clutch or torque converter, through which power is sent from the engine to the driveshaft in an automobile (motor car or motor truck).

   "Any vehicle with an internal combustion engine needs a transmission that can alter the gear ratio (engine RPM to wheel RPM) as the vehicle's speed changes."