Visual sightings of an unidentified flying object seemingly less than 500 feet away that show an appreciable angular extension and considerable detail.

NOTE: by this definition, a great many of the reported "sightings" do not qualify, as the majority of them are clearly beyond 500 feet from the observer making the report. (Instantly filtering out a large block of probable faulty data in that way, serves right off to validate the system. Cool. eh? UFOlogists practice better science than do most mainstream Human scientists! You gotta love it.)

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, scorched or otherwise affected vegetation, or a chemical trace.

UFO encounters in which an animated creature is present. These include humanoids, robots, and humans who seem to be occupants or pilots of a UFO.

The problem with this definition is that it assumes an ET (or UFO pilot) must always be both corporeal and visible to Human perceptions. As we know, this isn't always the case.

A UFO event in which a human is abducted by a UFO or its occupants.

Hynek's erstwhile associate Jacques Vallee argued in the Journal of Scientific Exploration that a CE4 should be described as "cases when witnesses experienced a transformation of their sense of reality", so as to also include non-abduction cases where absurd, hallucinatory or dreamlike events are associated with UFO encounters.

The film The Fourth Kind makes reference to this ranking.

A UFO event that involves direct communication between aliens and humans. This type of close encounter was named by Steven M. Greer's CSETI group and is described as bilateral contact experiences through conscious, voluntary and proactive human-initiated cooperative communication with extraterrestrial intelligence.

Death of a human or animal associated with a UFO sighting, although some might consider this as a more severe example of a second-kind encounter.

The creation of a human/alien hybrid, either by sexual reproduction or by artificial scientific methods.

Having a long term relationship with one or more ETs, or being revisited multiple times.
Well known examples would be Whittley Strieber and Billy Meier, perhaps Barbara Marciniak, which also qualify as CE5 events, but here we are talking about true long-term relationships, where the Human is revisited continually over years.

External link: www.ufocasebook.com/2008/longtermrelationship.html

An active relationship between an ET group and a Human group, but working in "secret," not officially, or out in the open.

The "supposed" joint alien/human work at Area 51 (or Groom Lake) would qualify, or the facility at Dulce Base. The wild tale of "a Nazi base in Antarctica" would not qualify, since they are not aliens (though, perhaps not really Human, either).

(The photo used here is obviously the work of Photoshop or something like it. We apologise for that only in part. We thought it might be interesting to view what the Humans imagine is going on, rather than what is really going on. See the MIB Library Archives for the real photos, if your clearance is high enough.)

Offical Contact, between ET and Human Society.

Except in Science Fiction literature, this has not happened in recorded Human history.

ETs join Human society.

Such as joining the UN, hanging out in bars, or otherwise becoming a normal part of the daily Earthly experience.

Again, except in the literary style called Science Fiction, this has not happened in main-stream Human historical records.

It's the "CE" from which there is no returning; the utlimate (and last) close encounter. Humans and "ETs" are now all one society, all equal, all piers.