An Etymological Dictionary of Astronomy and Astrophysics

1. The apparent → direction of → gravity at the point of observation.
   

2. Being in a position or direction → perpendicular to the → plane of the → horizon. See also → normal.

Etymology (EN): Vertical, literally “of or at the vertex, directly overhead,” from M.Fr. vertical, from L.L. verticalis “overhead,” from L. vertex (genitive verticis) “highest point”

Etymology (PE): Hajin, from haj, variant hac “anything held vertical, erected in the manner of a spear” (Dehxodâ), may be from Proto-Ir. *hac- “to follow;” cf. Av. (+ *ā-) hac- “to stick to;” Mid.Pers. hâz- “to lead, guide;”
→ associate.