1. In general, a device that both modulates and demodulates signals. 2. In computer communications, a device used for converting digital signals into, and recovering them from, quasi-analog signals suitable for transmission over analog communications channels. Note: Many additional functions may be added to a modem to provide for customer service and control features. Synonym signal conversion equipment. 3. In FDM carrier systems, a device that converts the voice band to, and recovers it from, the first level of frequency translation.