A gremlin is a mischievous folkloric creature invented at the beginning of the 20th century to originally explain malfunctions in aircraft and later in other machinery and processes and their operators. Depictions of these creatures vary widely. Stories about them and references to them as the causes of especially inexplicable technical and mental problems of pilots were especially popular during and after World War II. There is evidence of earlier RAF reference in the 1920s to a lowly menial person, in other words a low-ranking officer or enlisted man saddled with oppressive assignments.