Mirage
A mirage was a visual illusion of an object or individual which was not in fact present. 

In 1969, flight control dismissed  John Christopher's sighting of the , saying "We don't track mirages." ()

Mirages were traditionally associated with deserts. Because of that, "mirage" was also one of the words that came to Leonard McCoy's mind when he heard the Enterprise was about to cross a star desert in 2267. ()

In 2268, Doctor Leonard McCoy and Spock experienced what Spock identified as a mirage while on Minara II. ()

Also in in 2268, while on , James T. Kirk looked out of what appeared to be the viewing port of the Enterprise and briefly saw a number of people before a starfield appeared. He speculated that the people might be a mirage, existing only in the mind. ()

The word mirage was also mentioned in the Human song "". ()

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Physics
Psychology