Source : Webster's Revised Unabridged Dictionary (1913)

Aphelion \A*phel"ion\ (?; 277), n.; pl. {Aphelia}. [Gr. ? + ?
   sun.] (Astron.)
   That point of a planet's or comet's orbit which is most
   distant from the sun, the opposite point being the
   perihelion.

Source : WordNet®

aphelion
     n : apoapsis in solar orbit; the point in the orbit of a planet
         or comet that is at the greatest distance from the sun
         [ant: {perihelion}]
     [also: {aphelia} (pl)]