A UFO ( Unidentified Flying Object ) is any object flying in the sky which can not be identified by the person who sees it . An unidentified flying object ( usually abbreviated to UFO or U.F.O. ) is any unusual apparent object or phenomenon in the sky whose cause can not be identified by the observer , or ( in a narrower definition ) by investigators ; though in popular usage it more loosely means alien spacecraft , being one explanation ( among several ) offered for such sightings .