Neat interview with Steven Spielberg during filming on Close Encounters of the Third Kind about why gu'b'ment didn't care to discuss them (fear of confusion over ICBM being one aspect). Close Encounters filmed quickly after Jaws - Spielberg STILL wearing his rope bracelet from JAWS location filming on Martha's Vineyard. Good look. WATCH
As alluded to in the video, the "third kind" terminology comes from a system of classification codified by astronomer and UFO researcher J. Allen Hynek. The early CEOT3K posters listed the encounter 'kinds".