E.T. was an alien character in a popular Earth movie from 1982 called E.T. the Extra-Terrestial.
In 1996, when Rain Robinson at the Griffith Observatory picked up gamma radiation from the nacelles of the USS Voyager and contacted Henry Starling, Starling referred to the source of the gamma radiation as E.T., jokingly asking "Do you think E.T. likes Chateau Coeur?" (VOY: "Future's End")