With regards to UFO's and UFOlogy, a Close Encounter simply means to witness an Unidentified Flying Object (UFO).
Well, it seems as with everything in the world of UFOlogy that there are conflicting points of view regarding, “Close Encounters.” The, “Old school” train of thought relies wholly on Hynek's interpretation and recognises only the original three types of, “Close Encounter.” While some recognise these as well as a Close Encounter of the fourth, fifth and sixth kind, with some recognizing all 6 along with the sub-types.
The original terminology and the system used to qualify and classify these encounters was first proposed and introduced by UFO researcher and astronomer, “J. Allen Hynek.” It was in contained in his book, “The UFO Experience : A Scientific Study,” which was originally released way back in 1972.
It's also worth noting that UFO sightings more than 500 feet from the witness are classified as "Daylight Discs", "Nocturnal Lights" or "Radar/Visual Reports". Sightings within about 500 feet are sub-classified as various types of "close encounter." Close Encounter Of The First Kind: Aerial objects that are not attributable to human technology and/or a sighting of one or more unidentified flying objects; these include flying saucers, orbs, balls of light, cylinders etc.
Close Encounter Of The Second Kind: An observation of a UFO and associated physical effects from the UFO. 3-D physical tracings, or remnants, of something apparently not of this world/dimension , it would appear that Crop Circles would fall into this category, BUT only if the crop circle is found in the vicinity of a UFO sighting.
Also included in a Close Encounter of the Second kind is any damage to the terrain, interference to televisions, radios and internal combustion engines, frightened or panicked animals, heat, radiation, human paralysis etc. Close Encounter Of The Third Kind: The Third kind is when you not only encounter a UFO, but when you can also see the entities inside the UFO.