What is UFO?
It is most probably defined as any apparent anomaly in the sky,that is unidentifiable as known object is known as UFO(unidentified flying object).
First UFO Sighting:-
On June 24,1947 a pilot name Kenneth Arnold,saw many objects glowing and flying around the Mt. Rainier mountains at 10,000 feet like a "saucer " on water.
He said that the object were approx 50 ft. long and travelling at very high speed i.e. 1700 miles/h.He named it 'The Flying Saucer's'.Govt. first said that it will be might be a bird,or some aircraft but he refused it by saying that a 'bird cannot fly at a speed of 1700 miles/h .
After 3 days of Kenneth Arnold sighting there were 17 more no. of sighting observed.Around a week the saucer's sighting increases much more.
Second UFO Sighting:-
The Washington D.C. Sighting 1952-

Several regulations made to prevent investigators to give any news about UFO if they do not provide an explanation.
UFO's have gave rise to researchers,including the 'National Investigations committee on Aerial Investigations'(NICAP) in 20th century,the Mutual UFO Network(MUFON),and the Center of UFO studies(CUFOS).
The ufology is based to describe the collective efforts of those who study reports and associated evidence of unidentified flying objects.