Frequency-division multiplexing (FDM) is a form of signal multiplexing where multiple baseband signals are modulated on different frequency carrier waves and added together to create a composite signal.
FDM can also be used to combine multiple signals before final modulation onto a carrier wave. In this case the carrier signals are referred to as subcarriers: an example is stereo FM transmission, where a 19 kHz subcarrier is used to separate the left-right difference signal from the central left-right sum channel, prior to the frequency modulation of the composite signal.
Where frequency division multiplexing is used as to allow multiple users to share a physical communications channel, it is called frequency division multiple access (FDMA).
FDMA is the traditional way of separating radio signals from different transmitters.
The analog of frequency division multiplexing in the optical domain is known as wavelength division multiplexing.