Definitions about aliens and UFOs

Many UFO researchers argue that UFOs have appeared throughout history. There are many myths, legends, and stories that tell of strange things seen in the sky or beings who came from the sky to help humans develop civilization.
Because modern scholars cannot directly check the facts of these stories, it is impossible to determine if these are accurate reports of true events. Most ufologists, therefore, concentrate on studying UFO reports beginning in this century. Basing our study of such events on a common set of clear and accurate definitions seem to be a prerequisite for a serious and clear analysis.