Unidentified flying object (commonly referred to as UFO or U.F.O.) is the term used most commonly in reference to an aerial phenomenon that cannot be indentified by those who are witnessing the object. The term was first coined by United States Air Force (USAF) Captain Edwards Ruppelt in 1952 to replace the more common, at that time, term 'Flying Saucer'.
Although the letters "UFO" are usually pronounced seperately, some people pronounced them as one word, "you-foe."
Many UFOlogists use the term to describe only those objects that remain unidentified after investigation. Though it is used very looseing by many people to describe any object they cannot immediately identify, whether it has been investigated or not.
UFO is also used, normally, as a term for Extra-Terrestial (Alien) spacecraft and many discussion on the subject revolve around this assumption. UFO enthusiasts and devotees have created organizations and religious cults use the term UFO in the adopted extraterrestrial sense, and in general the UFO concept has evolved into a prominent mythos in modern culture.
Many UFOlogists and investigators now use the term Anomalistic Observational Phenomena (AOP) or Unidentified Aerial Phenomenon (UAP), this is mainly due to the nature of what UFO has become attributed with and thus avoid confusion.
Another popular term used for UFO is OVNI, although this is mainly used in those places whose language is Spanish, Portuguese, French, and Italian (Objeto Volador No Identificado, Objeto Voador Não Identificado, Objet volant non identifié or Oggetto Volante Non Identificato).