Definition of "UFO"

After the case had been reported in the newspapers, Mrs. Gracindo de Souza, wife of a member of the local stock exchange, told police that she and her daughter had been driving down Alameda Sao Boaventura when they had seen an[sic] UFO hovering over the clearing where the bodies were later discovered.

1967, Brad Steiger, Joan Whritenour, “Someone Up There May Not Like Us”, in Flying Saucers Are Hostile‎, Tandem Publishing, published 1975, page 13

To my way of thinking, there is every bit as much evidence for the existence of UFOs as there is for the existence of God. Probably far more. At least in the case of UFOs there have been countless taped and filmed—and, by the way, unexplained—sightings from all over the world, along with documented radar evidence seen by experienced military and civilian radar operators.

2004, George Carlin, “THEY CAME FROM OUT OF THE SKY”, in When Will Jesus Bring the Pork Chops?‎, New York: Hyperion Books, page 12

UFOs differ from WIPs in that a WIP is a project one is actively pursuing, while a UFO runs the risk of never getting finished.

2007, Kerry Wills, The Close-knit Circle: American Knitters Today, page 107