1. Short for modulator/demodulator. A communications device that converts between digital data from a computer or terminal and analog audio signals that can pass through a standard telephone line. Because the telephone system was designed to handle voice and other audio signals and a computer processes signals as discrete units of digital information, a modem is necessary at both ends of the telephone line to exchange data between computers. At the transmit end, the modem converts from digital to analog audio; at the receiving end, a second modem converts the analog audio back to its original digital form. In order to move a high volume of data, high-speed modems rely on sophisticated methods for “loading” information onto the audio carrier—for example, they may combine frequency shift keying, phase modulation, and amplitude modulation to enable a single change in the carrier’s state to represent multiple bits of data. In addition to the basic modulation and demodulation functions, most modems also include firmware that allows them to originate and answer telephone calls. International standards for modems are specified by the International Telecommunications Union, or ITU. Despite their capabilities, modems do require communications software in order to function. See also amplitude modulation, frequency modulation, quadrature amplitude modulation. Compare digital modem.
2. Any communications device that acts as an interface between a computer or terminal and a communications channel. Although such a device may not actually modulate or demodulate analog signals, it may be described as a modem because a modem is perceived by many users to be a black box that connects a computer to a communications line (such as a high-speed network or a cable TV system). See also digital modem.
(acronym for modulator/demodulator) Device for transmitting computer data over telephone lines. Such a device is necessary because the digital signals produced by computers cannot, at present, be transmitted directly over the telephone network, which uses analog signals. The modem converts the digital signals to analog, and back again. Modems are used for linking remote terminals to central computers and enable computers to communicate with each other anywhere in the world.
(From MOdulate-DEModulate) A communications device used to connect computers by a telephone line.
Modulator/Demodulator. A device that transforms digital signals into audio tones for transmission over telephone lines, and does the reverse for reception.

To send (as data) via a modem