Sightings of UFO's are invariably fleeting or from a distance. It seems to me that if approached and investigated, most UFO's are resolved into something more terrestrial and therefore cease to be 'unidentified' and are not reported (who reports KFO's? duh!).
There will never be clear footage of a UFO because clear footage would allow it to be identified and therefore not be a UFO.
My understanding is that 'UFO' is standard terminology in aviation. My plane from Beijing was delayed because of a UFO in the airspace. It simply means what it says - unidentified flying object. Could be a plastic bag - it's still a UFO if you're not sure what it is.