Many UFOs are experimental aircraft from ours and other countries, some are optical illusions (from the observers point of view), many have been reported or observed only in the minds of attention seekers, some have been a weather related phenomenon, and some have been demons going around creating panic in the minds of the public but also to give a "reasonable" excuse when the rapture happens and millions of people are missing.
or weather balloons, swamp gas, pieces of debris burning up in our atmosphere, …….etc.

from The American Heritage® Dictionary of the English Language, 5th Edition.

    noun An unidentified flying object.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

    initialism unidentified flying object
    noun An unidentified flying object.
    noun informal An alien spacecraft.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

    noun an (apparently) flying object whose nature is unknown; especially those considered to have extraterrestrial origins



of the Books below maybe checked out while some are free for the taking other must be bought

This craft is the prototype of a giant `flying saucer' designed to revolutionize air transport. Designed by British firm, Airship Industries, the Skyship was planned to cruise at about 100 miles an hour at an altitude of 5000 feet.