(covers information from several alternate timelines)
A supposed "mirage"
A mirage was a visual illusion of an object or individual which was not in fact present.
In 1969, flight control dismissed Captain John Christopher's sighting of the USS Enterprise, saying "We don't track mirages." (TOS: "Tomorrow is Yesterday")
Mirages were traditionally associated with deserts. Because of that, "mirage" was also one of the words that came to Doctor Leonard McCoy's mind when he heard the Enterprise was about to cross a star desert in 2267. (TOS: "The Squire of Gothos")
In 2268, McCoy and Spock experienced what Spock identified as a mirage while on Minara II. (TOS: "The Empath")
Also in 2268, while on Gideon, 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. (TOS: "The Mark of Gideon")
The word mirage was also mentioned in the Human song "Sabotage". (Star Trek)