Transmission

1. The dispatching, for reception elsewhere, of a signal, message, or other form of information.

2. The propagation of a signal, message, or other form of information by any means, such as by telegraph, telephone, radio, television, or facsimile via any medium, such as wire, coaxial cable, microwave, optical fiber, or radio frequency.

3. In communications systems, a series of data units, such as blocks, messages, or frames.

4. The transfer of electrical power from one location to another via conductors.

Source: from Federal Standard 1037C and from MIL-STD-188

With regard to automobiles, a transmission is a system which allows variable selection of application of torque to the engine. This can be done manually or automatically.