J. Allen Hynek Classification of UFO phenomena:

Categories for distant sightings (500 meters or more):
Nocturnal Lights (NL): Anomalous lights in the night sky
Daylight Discs (DD): Abnormal objects in daylight (not necessarily disk-shaped)
Radar / visual cases (RV): Abnormal objects is simultaneously visually observed and recorded by radar.
Categories for close encounters (CE stands for close encounter, close encounters with German.)
Close encounters of the first kind (CE-1): Unidentified flying object is observed from less than 500 meters away.
Close encounters of the second kind (CE-2): Unidentified flying object generates physical interactions with the environment (such as burns, imprints in the ground, electrical equipment malfunctions, extremely strongmagnetic fields).
Close Encounters of the Third Kind (CE-3): In addition to flying objects landed alien inmates are observed,which do not attempt to communicate and to feel disturbed by the presence of the witnesses seem more likely.
Close encounters of the fourth kind (CE-4): Witnesses feel that they have been abducted by UFO occupants and medically examined.