A gremlin was a fictional monster created by 20th century British Royal Air Force pilots to explain why their airplanes were malfunctioning.
In 2151, Captain Jonathan Archer asked Sub-Commander T'Pol if she was still chasing gremlins when she suspected that supposedly mythical microsingularities were responsible for a collision with a Tesnian starship. (ENT: "Shuttlepod One")
In 2152, Archer blamed gremlins for a noise underneath the deck plating that wouldn't go away. T'Pol was confused by the statement, but Archer explained it. (ENT: "Dead Stop")
It is strange that T'Pol seemed to be familiar with gremlins in November 2151, yet was unaware of them only a few months later. Conceivably, she simply forgot or did not retain the information due to a disinterest in mythical creatures.
In 2376, Captain Kathryn Janeway suggested to Chief Engineer B'Elanna Torres that gremlins were the cause of several malfunctions aboard the USS Voyager. (VOY: "Live Fast and Prosper")
Similarly, Tom Paris said "Who says gremlins in the engine are a myth?" after Fantome and his people shut down the power on Valen and Bosaal's ships in 2377. (VOY: "The Void")
One of William Shatner's most well-remembered pre-Star Trek roles involved him facing a gremlin in the The Twilight Zone episode "Nightmare at 20,000 Feet".