Why they are called UFOs:

Unidentified Flying Objects or UFO's refer to any object in the sky that is not readily identifiable. Strange sky apparitions have been reported throughout history. Some were identified overtime as bright comets or meteors,or other atmospheric phenomena. However, others were still unexplained and are simply known as Unidentified Flying Objects.

After World War II, there were numerous reported sightings in the United States. Kenneth Arnold, a business man, claimed that on June 1947 he saw nine flying objects across Washington sky. He later described them as ‘flying like saucers” which lead to the phrase, "flying disk" and "flying saucers".

This was quickly followed by several sightings in other countries as well as the United States. However, the one sighting  that stood out the most was reported by a crew of United Airlines who claimed that they saw the same 9 flying saucers, thus supporting Kenneth Arnold’s claim.

These reported sightings immediately caught media attention and the government became more concerned. After a month from the initial sighting, the US government launched an investigation into flying saucers. The investigation proved that reported sightings were real and there were indeed flying objects. Technical and intelligence divisions of the US Air Force supported these findings. These investigations led to “Project Blue Book”, to further study and understand flying saucers.

It was in 1952 when the term "flying saucer" was officially replaced with UFO or Unidentified Flying Objects. In 1954, the director of Project Blue Book issued a statement formally defining UFO as ANY airborne flying object that has aerodynamic characteristics and unusual features and doesn't conform to known missile or aircraft types. It also means objects that aren't known or can't be identified positively as familiar objects. UFO's were later categorized based on their features: 

1. Objects that have fast moving lights,and rapidly change route. 
2. Disk-shaped objects that don't have audible or visible propulsion.
3. Large triangular objects that have the same light pattern.
4. Cigar-shaped objects that have lighted windows.

Along with visual sightings, UFO's were linked with physical evidence. The most popular evidence are Crop Circles or ground impressions. Experts believe that UFO's leave dessicated or burned soil with metallic traces.

Today, there are still UFO sightings throughout the world that continue to amaze all kinds of people. Until they are identified or come in contact with experts, they will remain Unidentified Flying objects.