While technically a UFO refers to any unidentified flying object, in modern popular culture the term UFO has generally become synonymous with alien spacecraft. however the term ETV (ExtraTerrestrial Vehicle) is sometimes used to separate this explanation of UFOs from totally earthbound explanations.
Proponents argue that because these objects appear to be technological and not natural phenomenon, and are alleged to display flight characteristics or have shapes seemingly unknown to conventional technology, the conclusion is then that they must not be from Earth.
Though UFO sightings have occurred throughout recorded history, modern interest in them dates from World War II (see foo fighter), further fuelled in the late 1940s by Kenneth Arnold's coining of the term flying saucer and the Roswell UFO Incident. Since then governments have investigated UFO reports, often from a military perspective- and UFO researchers have investigated, written about, and created organisations devoted to the subject.
One such investigation, The UK's Project Condign report, notes that Russian, Former Soviet Republics, and Chinese authorities have made a co-ordinated effort to understand the UFO topic and that State military organisations, particularly in Russia, have done "considerably more work (than is evident from open sources)" on military applications which have stemmed from their UFO research. The report also noted that "several aircraft have been destroyed and at least four pilots have been killed 'chasing UFOs'.