Definition
An electronic device that converts digital data from a computer into an analog signal suitable for transmission over a communication line, and converts incoming analog signals back into digital data the computer can use.
Plain English
A device that lets a computer send and receive information over a phone line or similar circuit by translating between the computer's digital signals and the analog signals the line can carry.
Context Anchor
Seen in avionics, data link, aircraft communications, and maintenance-manual discussions involving electronic data transfer.
Derivation
The name is built from the two jobs the device performs: it modulates (shapes a carrier signal to carry data going out) and demodulates (recovers the data from an incoming signal). Knowing this makes the function obvious from the name.
Why Pilots Care
A modem may be part of the equipment path that moves flight plans, weather data, position reports, or maintenance information. If that path fails, the aircraft may lose a data service even though the airplane itself is still flyable.
Analogy
A modem is like a translator between two forms of communication: one form the computer understands, and one form that can travel through a wire or radio link.
Example Sentence 1
The aircraft's ACARS unit uses a modem to send position reports to the airline's dispatch office.
Example Sentence 2
Maintenance used the modem connection to upload the latest navigation database to the flight management computer.