Plain English Meaning
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.
Full 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.
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.