UFO Terminology

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Unidentified Flying Object(UFO)
This is a popular term for any aerial phenomenon that is not readily identified.

Identified Flying Object(IFO)
These are aerial objects that can be identified on sight or by later investigation.

Unidentified Submerged Object(USO)
This is an object found underwater that can't be identified by sight or later investigation.

Close Encounter
A close encounter is an event where a person witnesses a UFO. There are five different types of close encounters:

Close Encounter of the First Kind
This is a sighting of one or more unidentified flying object whether it be a ball of light, 'flying saucer' or another unidentified aerial object.

Close Encounter of the Second Kind
This is the sighting of one or more unidentified flying object and the physical effects of the object.
    * Heat or radiation
    * Damage to terrain
    * Human paralysis
    * Frightened animals
    * Interference with engines or TV or radio reception.
    * Lost Time: a gap in one's memory associated with a UFO encounter.

Close Encounter of the Third Kind
Observation of what Josef Allen Hynek termed "animate beings" observed in association with a UFO sighting. UFO researcher Ted Bloecher proposed seven subtypes for the close encounters of the third kind in the Hynek's scale.
    A: An entity is observed only inside the UFO
    B: An entity is observed inside and outside the UFO
    C: An entity is observed near to a UFO, but not going in or out.
    D: An entity is observed. No UFOs are seen by the observer, but UFO activity has been reported in the area at about the same time
    E: An entity is observed. But no UFOs are seen and no UFO activity has been reported in the area at that time
    F: No entity or UFOs are observed, but the subject experiences some kind of "intelligent communication"
    G: Abduction (same as close encounter of fourth kind)

Subtypes D, E, and F may be unrelated to the UFO phenomenon.

Close Encounter of the Fourth Kind
A human is abducted(taken against will) by a UFO or its occupants.

Close Encounter of the Fifth Kind
These encounters are joint, bilateral contact events produced through the conscious, voluntary and proactive human-initiated or cooperative communication with Extraterrestrial intelligence.

Identified Alien Craft (IAC)
This is a tern allegedly used by organisations/groups of people in the know to label UFOs, the acronym UFO is not used by these organisations/groups.