In ufology, a close encounter is an event in which a person witnesses an unidentified flying object. This terminology and the system of classification behind it was started by astronomer and UFO researcher J. Allen Hynek, and was first suggested in his 1972 book The UFO Experience: A Scientific Inquiry. He introduced the first three kinds of encounters; more sub-types of close encounters were later added by others, but these additional categories are not universally accepted by UFO researchers, mainly because they depart from the scientific rigor that Hynek aimed to bring to ufology.
Sightings more than 500 feet (150 m) from the witness are classified as "Daylight Discs," "Nocturnal Lights," or "Radar/Visual Reports." Sightings within about 500 feet are subclassified as various types of "close encounter." Hynek and others argued a claimed close encounter must occur within about 500 feet to greatly reduce or eliminate the possibility of misidentifying conventional aircraft or other known phenomena.