mirage

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An optical phenomenon that creates the illusion of water, often with inverted reflections of distant objects, and results from distortion of light by alternate layers of hot and cool air. Also called fata morgana.
2. n. Something illusory or insubstantial.

Century Dictionary and Cyclopedia

1. n. An optical illusion due to excessive bending of light-rays in traversing adjacent layers of air of widely different densities, whereby distorted, displaced, or inverted images are produced. The requisite change in density arises only near the earth's surface, and the hot shining of the sun seems to be an invariable antecedent, The mirage of the desert presents an appearance of objects reflected in a surface of water; in this case the heated earth rarefies the air in the lower strata faster than it can escape, and the flatness of the ground conduces to the maintenance of the resulting abnormal distribution of density. Displacement by mirage is commonly vertical, but is lateral when the density-gradi ent is more or less inclined to the vertical. Looming and fata Morgana are species of mirage. See these words.
2. n. Hence Deceptiveness of appearance; a delusive seeming; an illusion.

Wiktionary

1. n. An optical phenomenon in which light is refracted through a layer of hot air close to the ground, giving the appearance of there being refuge in the distance.
2. n. figuratively An illusion.

GNU Webster's 1913

1. n. An optical effect, sometimes seen on the ocean, but more frequently in deserts, due to total reflection of light at the surface common to two strata of air differently heated. The reflected image is seen, commonly in an inverted position, while the real object may or may not be in sight. When the surface is horizontal, and below the eye, the appearance is that of a sheet of water in which the object is seen reflected; when the reflecting surface is above the eye, the image is seen projected against the sky. The fata Morgana and looming are species of mirage.

WordNet 3.0

1. n. an optical illusion in which atmospheric refraction by a layer of hot air distorts or inverts reflections of distant objects
2. n. something illusory and unattainable