Modem

From Wistful.net

Modem stands for Modulator/Demodulator (hence the use of the first three letters of each word used to create one). It connects a computer to another over POTS (telephone) lines and allows the two to transfer serial data between each other. So send data it is first modulated into an audible tone and sent over the line to the other side which demodulates it and gives that data to the computer (or other terminal equipment) as serial data. Most modems these days are used to connect computers to ISPs so they can check their spam and surf for pr0n.