A Gremlin is a mythological creature similiar to a fairy or goblin which originated during the World Wars, in which they were said to cause havoc on aircrafts - a trait which they have been continually given since their creation, though in time their destructive nature has come to include almost any type of technology.
Gremlins have become a staple of fiction and inspired many mischievous and/or malicious beings throughout the ages - the most famous of which are arguably the titular monsters of the comedy-horror "Gremlins" (though these monsters bared little in common with the folkloric creatures).
Although not popularly used until after World War II, the myth of the Gremlin seems to be older, the earliest example going back to the 1920s - it may of been a way for airmen to explain faults in aircraft or malfunctions, which were often seen as inexplicable and over time they became a part of the public imagination.
At least one theory as to the origins of Gremlins is that they were fairy folk who helped mankind to achieve the technological marvels of the Industrial Revolution but became spiteful after not receiving any acknowledgement for their work as well as the destruction of magical thought that came with mass-industrialization, thus ever since that fateful day the Gremlins have sought to disrupt and attack any and all technology (it is worth noting this theory is in many ways a fantastical telling of the Luddite movement, who were a very real human organization that took to violently attacking machinery they saw as endangering their current way of work).