While proponents of the extraterrestrial hypothesis (ETH) suggest that these unexplained reports are of alien spacecraft, the null hypothesis cannot be excluded that these reports are simply other more prosaic phenomena that cannot be identified due to lack of complete information or due to the necessary subjectivity of the reports.
While UFOs have been the subject of extensive investigation by various governments and although a few scientists have supported the extraterrestrial hypothesis, almost no scientific papers about UFOs have been published in peer-reviewed journals.
Studies have established that the majority of UFO observations are misidentified conventional objects or natural phenomena—most commonly aircraft, balloons, noctilucent clouds, nacreous clouds, or astronomical objects such as meteors or bright planets with a small percentage even being hoaxes.
Between 5% and 20% of reported sightings are not explained, and therefore can be classified as unidentified in the strictest sense.
Such sightings throughout history often were treated as supernatural portents, angels, or other religious omens.
The Oxford English Dictionary defines a UFO as "An unidentified flying object; a 'flying saucer'." The first published book to use the word was authored by Donald E. He wrote, "Obviously the term 'flying saucer' is misleading when applied to objects of every conceivable shape and performance.
Most UFOs are later identified as conventional objects or phenomena (e.g., aircraft, weather balloons, clouds).
Some of them are not identified, either because of lack of evidence or because no conventional explanation can be found despite extensive evidence.
At the time, he claimed he described the objects flying in a saucer-like fashion, leading to newspaper accounts of "flying saucers" and "flying discs".
and because of the public and media ridicule associated with the topic, some investigators prefer to use such terms as unidentified aerial phenomenon (or UAP) or anomalous phenomena, as in the title of the National Aviation Reporting Center on Anomalous Phenomena (NARCAP).