The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

aphelion
    n 1: 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]