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.
Close Encounters of the First kind: Visual sightings of an unidentified flying object seemingly less than 500 feet away that show an appreciable angular extension and considerable detail.11
Close Encounters of the Second kind: A UFO event in which a physical effect is alleged. This can be interference in the functioning of a vehicle or electronic device; animals reacting; a physiological effect such as paralysis or heat and discomfort in the witness; or some physical trace like impressions in the ground, scorching or effecting vegetation, or a chemical trace.12
Close Encounters of the Third kind: UFO encounters in which an animated creature is present. These include humanoids, robots, and human who seemed to be occupants or pilots of a UFO..13
Close Encounters of the Fourth kind: Human is abducted by a UFO or its occupants.
Close Encounters of the Fifth kind: Named by Steven M. Greer’s CSETI group, these purported encounters are joint, bilateral contact events produced through the conscious, voluntary and proactive human-initiated or cooperative communication with extraterrestrial intelligence. This is very similar to some “contactees” of the 1950s who claimed regular communication with benevolent aliens.