Well there are UFOs as they stand for Unidentified Flying Objects. Which basically means anything flying that can't be identified. So it could refer to a balloon floating by to a regular airplane to an alien spacecraft (flying saucer).
There were two different television series programs entitled UFO. One, the British UFO produced by Science-fiction mogul Gerry Anderson ( who needs no introduction) dealt with the activities of the...
UFO means "unidentified flying object", which means you look up and see something and don't know what it is. In that sense UFOs are real. There are plenty of natural and man-made thing in the sky...