Source : Webster's Revised Unabridged Dictionary (1913)

Mirage \Mi`rage"\, n. [F., fr. mirer to look at carefully, to
   aim, se mirer to look at one's self in a glass, to reflect,
   to be reflected, LL. mirare to look at. See {Mirror}.]
   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.

         By the mirage uplifted the land floats vague in the
         ether, Ships and the shadows of ships hang in the
         motionless air.                          --Longfellow.

Source : WordNet®

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