From Computer History Wiki
A modem (short for "modulator - demodulator") is a generic term for a device which transforms a signal in one form into another form. The best-known form of modems are those which transform a digital bit stream into analog signals.
For many years, from the 1960s until around the end of the century, modems which could be used to transmit data over telephone lines were the most common. With the rise of Internet access over cable TV, cable TV modems became common.