A purported flying saucer over a house in post-war suburban USA.
'Flying saucer' was a term coined following the reported sighting by Kenneth Arnold, on June 24, 1947, of nine luminous objects in the sky over Washington state. Arnold, a businessman and part-time deputy sheriff was piloting a private plane when he saw the unusual lights heading south from the direction of Mount Baker toward Mount Rainier at what he estimated was a supersonic speed. In describing to reporters later how the objects moved he said that they hugged the contours and appeared to undulate "like a saucer skipping over water." By the time, the story appeared in print his words had been distorted to the phrase "flying saucer." Arnold's sighting, and the great publicity it received, triggered the great saucer flap of 1947. Various theories have been put forward to account for the incident, including an especially plausible one which involves the little understood phenomenon of earthlights.
'Flying disk' was an alternative description, commonly used between 1947 and 1952, for the phenomenon also known as flying saucers. It appears to have been the preferred term in military circles until it was superseded by 'unidentified flying object' at the start of Project Blue Book.
Alternatively, 'flying disk' was a popular description for the Avrocar experimental aircraft. Incidentally, there is no possibility that the Avrocar, despite its appearance, could have been responsible for any UFO reports, since it only flew at test sites and never at more than a couple of meters above the ground.