Dr. J. Allen Hynek isn’t exactly a household name, but he is one of science’s most renowned astronomers and ufologists (yes, that’s a word; it refers to someone who studies UFOs). He worked for the US government in the 1940s and 50s to determine the validity of reported UFO sightings, and is responsible for coining the classification system upon which the film is titled. You can call him the godfather of UFO research.
A "Close Encounter of the First Kind" is the 4th entry on Hynek's scale of UFO sightings, meaning someone visually witnessed a UFO "less than 500 feet away that shows an appreciable angular extension and considerable detail." A "Close Encounter of the Second Kind" is a UFO even with an alleged physical event. "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." A "Close Encounter of the Third Kind" is a UFO encounter where an animated creature is present, as happens in Spielberg's film.
To ensure the film properly reflected reported beliefs about alien sightings, Spielberg hired Hynek to be an advisor on Close Encounters of the Third Kind (1977). Many elements of the film’s script were drawn from real-life reports of UFO sightings around the country. Hynek was brought in to ensure the film represented these reports as accurately as possible.
Hynek has a brief cameo in the film. As the aliens disembark the mothership, he is seen wearing glasses and smoking a pipe.