An external or internal device connected to a computer for transmitting and receiving signals over various communication lines. It is an abbreviation for "modulator-demodulator," which indicates the initial principle of operation of this device: converting the digital signal received from the computer into analog form for transmission over wired networks and vice versa, converting the received signal from analog form into digital. With the advancement of technology, it is primarily understood as a device for generating incoming and outgoing signals from the computer.