(covers information from several alternate timelines)
A mirage was a visual illusion of an object or individual which was not in fact present.
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. (TOS: "The Squire of Gothos")
Also in 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")