WordNet sense 1 (the fraction of radiant energy that passes through a substance):

transmittance

WordNet sense 2 (an incident in which an infectious disease is transmitted):

contagion, infection

WordNet sense 3 (the gears that transmit power from an automobile engine via the driveshaft to the live axle):

transmission system

WordNet sense 4 (the act of sending a message; causing a message to be transmitted):

transmittal, transmitting