The Collaborative International Dictionary of English v.0.48:

Apex \A"pex\, n.; pl. E. Apexes; L. Apices. [L.]
   1. The tip, top, point, or angular summit of anything; as,
      the apex of a mountain, spire, or cone; the apex, or tip,
      of a leaf.
      [1913 Webster]

   2. (Mining) The end or edge of a vein nearest the surface.
      [U.S.]
      [1913 Webster]

   Apex of the earth's motion (Astron.), that point of the
      heavens toward which the earth is moving in its orbit.
      [1913 Webster] aphaeresis

WordNet (r) 3.0 (2006):

apex
    n 1: the highest point (of something); "at the peak of the
         pyramid" [syn: vertex, peak, apex, acme]
    2: the point on the celestial sphere toward which the sun and
       solar system appear to be moving relative to the fixed stars
       [syn: apex, solar apex, apex of the sun's way] [ant:
       antapex]