This could be a workable definition, but we'd need to be a bit specific about just who has to be reasonably certain. Your average alien-spaceship believer needs very little in order to be reasonably certain that any given light in the sky is not of natural or manmade origin. Skeptical investigators, on the other hand, tend to have a rather higher standard.In the lexicon of ufology the word UFO also has different meanings in different contexts. While it is clear that the word is meant to be applied to objects that appear to be extraordinary or out of this world, not all the objects in UFO reports turn out to be UFOs. Therefore for the purpose of investigation, the objects in UFO reports are not classed as UFOs until the reports have been investigated and all known natural or manmade objects have been ruled out with reasonable certainty.
This is better, but the problem remains of who is required to evaluate the evidence. Is it enough to be merely "technically capable", or should we require a certain level of actual critical thinking and skepticism in evaluating the evidence?'We can define the UFO simply as the reported perception of an object or light seen in the sky or upon land the appearance, trajectory, and general dynamic and luminescent behavior of which do not suggest a logical, conventional explanation and which is not only mystifying to the original percipients but remains unidentified after close scrutiny of all available evidence by persons who are technically capable of making a common sense identification, if one is possible.'
Hynek himself started off his adventures with UFO reports as a fairly level-headed skeptic, but gradually shifted to a certain level of credulity as his base setting, from what I gather.