Wiktionary
n. (context telecommunications English) A separate signal carried on a main radio transmission, often carrying additional information such as voice or data.
Wikipedia
A subcarrier is a sideband of a radio frequency carrier wave, which is modulated to send additional information. Examples include the provision of colour in a black and white television system or the provision of stereo in a monophonic radio broadcast. There is no physical difference between a carrier and a subcarrier; the "sub" implies that it has been derived from a carrier, which has been amplitude modulated by a steady signal and has a constant frequency relation to it.
Usage examples of "subcarrier".
Without her, it would have eventually been picked up by the special subcarrier tracker used here as an antibugging security measure.
The only subcarriers of the Center's computer-to-satellite link were too old and noisy for any real communication but would still carry a simple pulse tone if the transmitter were aimed at the correct angle toward the master transmitter.