1. Modulator-demodulator is a device that converts binary data to audio-frequency-shift-keyed (AFSK) analog signals, and vice-versa. It is commonly used to interface computers and terminals with telephone lines and radio transceivers. It is also called modem.
2. It is a circuit or device, such as a biased diode or diode bridge, that can perform either modulation or demodulation.
See also, Modulation frequency