In ufology, a close encounter is an event in which a person witnesses an unidentified flying object (UFO) at relatively close range, where the possibility of mis-identification is presumably greatly reduced. This terminology and the system of classification behind it were first suggested in astronomer and UFO researcher J. Allen Hynek's book The UFO Experience: A Scientific Inquiry (1972). Categories beyond Hynek's original three have been added by others but have not gained universal acceptance, mainly because they lack the scientific rigor that Hynek aimed to bring to ufology.

Distant sightings more than 150 meters (500 ft) from the witness are classified as daylight discs, nocturnal lights, or radar/visual reports. Sightings within about 150 meters (500 ft) are sub-classified as various types of close encounters. Hynek and others argued that a claimed close encounter must occur within about 150 meters (500 ft) to greatly reduce or eliminate the possibility of misidentifying conventional aircraft or other known phenomena.

Hynek's scale became well known after being referenced in the classic sci-fi film Close Encounters of the Third Kind (1977), which is named after the third level of the scale. Promotional posters for the film featured the three levels of the scale, and Hynek himself makes a cameo appearance near the end of the film.^{[citation needed]}

Hynek devised a six-fold classification for UFO sightings.

The six levels are arranged according to increasing proximity:

UFO researcher Ted Bloecher proposed six sub-types for the close encounters of the third kind in Hynek's scale:

After Hynek's death in 1986, his colleague Jacques Vallee extended Hynek's classification system by two steps, specifically close encounters of the fourth and fifth kinds, as published in Vallee's book Confrontations: A Scientist's Search for Alien Contact (1990).

The Mutual UFO Network (MUFON) immediately adopted the extensions to the classification scale and has used them ever since.^{[citation needed]}