Real Dictionary

mirage
If you describe something as a mirage, you mean that it is not real or true, although it may seem to be.
countable noun: A mirage is something which you see when it is extremely hot, for example in the desert, and which appears to be quite near but is actually a long way away or does not really exist.

mirage in American English
an optical illusion in which the image of a distant object, as a ship or an oasis, is made to appear nearby, floating in air, inverted, etc.: it is caused by the refraction of light rays from the object through layers of air having different densities as
noun: an optical phenomenon, esp. in the desert or at sea, by which the image of some object appears displaced above, below, or to one side of its true position as a result of spatial variations of the index of refraction of air

mirage in British English
noun: an image of a distant object or sheet of water, often inverted or distorted, caused by atmospheric refraction by hot air