An abbreviation of Modulator/Demodulator, a device used to transmit/receive digital data via an analog communication channel. Common kinds of modems:

• acoustic modem -- the original kind of modem, where the analog channel is a telephone voice line. These days most commonly used for dial-up Internet access, once upon a time also heavily used for connecting to BBSes.
• ADSL modem -- connects to ADSL service, which takes advantage of the fact that the copper wires used for telephone landline service have a much greater bandwidth than is needed for just voice.
• cable modem -- connects to an Internet service provided by a cable TV company.

Modulation refers to any kind of technique whereby a carrier signal output by the transmitting end has its characteristics altered in some systematic way that can be detected by the receiving end, in order to convey information.

Most kinds of modems nowadays are not symmetrical--they are designed to communicate with special equipment belonging to a central service provider, not with a modem bought by another ordinary person-off-the-street. Acoustic modems still have symmetrical communication capability, but only at speeds up to 33.6kb/s (V.34). The higher 56kb/s speed (V.90) only works with special equipment at the provider end.

Most modern acoustic MoDems, so-called WinModems, do not contain the hardware necessary to perform all of this operation. Instead, they rely on the host CPU for much of the signal processing. Because the software to do this is part of the MoDem driver, and many vendors write drivers only for MicrosoftWindows, some WinModems cannot be operated under Linux.