apex apex /ˈeɪˌpɛks/

noun

1. () The highest point (of something)  ( acme , peak , vertex , apex )
   the highest point (of something)
    At the peak of the pyramid
   at the peak of the pyramid
   
   
2. The point on the celestial sphere toward which the sun and solar system appear to be moving relative to the fixed stars  ( apex , apex of the sun's way , solar apex )
   the point on the celestial sphere toward which the sun and solar system appear to be moving relative to the fixed stars