A gremlin is an imaginary creature that causes trouble in devices and systems of all kinds. During the Second World War, the term was used by British airmen to refer to ongoing trouble with aircraft in spite of mechanics' best efforts. Gremlins sometimes appear today in computer systems and networks.
Although gremlins never do their dirty work in plain sight, a gremlin is usually portrayed as a small troll-like creature. No one has ever seen one and caught it in the act of sabotaging an aircraft, radio transmitter, computer, robot, or other system. The instant any human looks for a gremlin, it vanishes, although evidence of its mischief may remain. Gremlins are particularly adept at causing intermittent malfunctions, which have been the bane of technicians and engineers for centuries.