Most people believe that a ufo is the same as a "space ship" or a vehicle from outer space. This is not true. "Space ships" is just one of several theories that eventually might explain some of the sightings.
Originally the word "ufo" was an abbreviation of unidentified flying objects.
We define the term ufo-phenomenon as a phenomenon in the sky, having the primary characteristic of not being explainable as a natural phenomenon even when investigated thoroughly by competent researchers.
Or put more simply: A ufo is something in the sky that we cannot explain after a serious investigation.
A number of theories exist that might explain the ufo phenomenon. Some of them are:
It is very important to consider that by all probability there are not just one theory that can explain all sightings. Maybe there are several, different phenomena hiding behind the term
"UFO".