Unidentified Flying Object

UFO was, during the 20th and 21st centuries, a human designation standing for Unidentified Flying Object. Many UFOs were believed by some of the general public, such as Eugene Jones and Maxwell Edison, to be alien spacecraft. Particularly in the 1950s and into the 1970s, the public often reported UFOs as flying saucers, crewed by "Martians".

Alien defence organisations such as UNIT and the Torchwood Institute and insiders like Henry van Statten knew that many UFOs did, as suspected, come from planets other than Earth.

References and appearances
In 1941, Chris Cwej thought that a secret weapon the Nazis were working on was a UFO. (PROSE: Just War)

In 1957, the Sixth Doctor and Melanie Bush investigated UFO sightings in the proximity of Hammerston, Louisiana and became involved in the production of the film Swamp of Horrors after attacking an actor in a rubber alien suit whom they mistook for a genuine alien. The Doctor, who used his pseudonym "Dr. John Smith", and Mel were assumed to be uncredited actors playing fictional characters by film critics. (PROSE: Swamp of Horrors (1957) - Viewing Notes)

In 1965, the Latter-Day Pantheon caused a UFO scare in New York City. (PROSE: Salvation)

UNIT noted frequent UFO activity centred around Tobias Vaughn's company, International Electromatics, and correctly suspected alien involvement. (TV: The Invasion)

In March 2006, a UFO, actually a Slitheen craft, crashed into Big Ben. The Slitheen family had staged this crash to provoke World War III, only to be defeated by the Ninth Doctor. (TV: Aliens of London/World War Three)

In 2007, the Tenth Doctor and Rose Tyler went undercover at Deffry Vale High School after Mickey Smith and locals of the borough first noticed UFO activity - "lights in the sky" - around the area, later shown to be of Krillitane origin. (WC: Tardisode 4, TV: School Reunion)

Torchwood 3 briefly went on alert after UFO sightings in Cardiff airspace. They discovered it was only an excursion by members of the non-threatening Arcateenians. (TV: Cyberwoman)

In her attempts to find the Tenth Doctor after meeting him on Christmas of 2007, Donna Noble investigated, among other things, UFO sightings. (TV: Partners in Crime)

In 2009, Sarah Jane Smith investigated some UFO activity which later turned out to be caused by a Sontaran known as Kaagh. (TV: The Last Sontaran)

In 2400, a crewman on an observation satellite orbiting Earth reported seeing hundreds of flying saucers (actually Daleks on transolar disks) on his scanner. A physician told him that flying saucers were a myth of the 20th century and advised sick leave. (COMIC: Invasion of the Daleks)