An Etymological Dictionary of Astronomy and Astrophysics

1. General: The highest point or level.
   

2. Mathematics: The highest point of a geometric figure or solid relative to some line or plane.
   

3. Astro.: Solar apex. The point on the celestial sphere toward which the Sun and the solar system are moving relative to the stars in our vicinity.

Etymology (EN): L. apex “summit, peak, tip,” probably related to apere
“to fasten, fix,” hence “the tip of anything”.

Etymology (PE): Cakâd “summit of a mountain; top, crown of the head, top of the forehead,” from Mid.Pers. cakât “summit,” cf. Skt. kakud-, kakuda- “peak, summit,” L. cacumen “top, point,” cumulus “heap.”