UFO - Unidentified Flying Object
An "unidentified flying object" is a term used to denote "a flying object that does not belong or known to our world". This is basically a flying object that cannot be identified as an airplane, spaceship, helicopter, balloon, kite, or any other flying object that belongs to the Earth.
Sometimes a UFO is referred as an alien spacecraft. UFO sightings have been infrequently reported in the past few decades and raises questions about "life" in other planets. Ufologists, scientists who study about UFOs, have theories regarding them, but in the absence of any proof, these theories are just educated guesses.