The Collaborative International Dictionary of English v.0.48:

gegenschein \ge"gen*schein`\ (g[=a]"g[i^]n*sh[imac]n`) n.
   [German: counterglow.]
   A faint patch of light in the night sky that appears opposite
   the sun; a reflection of sunlight by micrometeoric material
   in space.

   Syn: counterglow.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Counterglow \Coun"ter*glow`\, n. (Astron.)
   An exceedingly faint roundish or somewhat oblong nebulous
   light near the ecliptic and opposite the sun, best seen
   during September and October, when in the constellations
   Sagittarius and Pisces. Its cause is not yet understood.
   Called also Gegenschein.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

gegenschein
    n 1: a faint spot of light in the night sky that appears
         directly opposite the position of the sun; a reflection of
         sunlight by micrometeoric material in space [syn:
         counterglow, gegenschein]