An Etymological Dictionary of Astronomy and Astrophysics

1. One of the principal lines through the center of a figure or a solid, especially, the line which divides the figure or solid symmetrically.
   
2. A fixed line along which distances are measured or to which position are referred.

Etymology (EN): L. axis “axle, pivot,” akin to O.E. eax “axis, axle,” Gk. axon “axle,” Skt. aksa- “axle, axis, beam of a balance;” PIE base *aks- “axis.”

Etymology (PE): Âsé, from Pers. dialects: Qâyeni asak “the shaft connecting the plough to the yoke,” Lori esi “a pillar
(used to put up a tribal tent),” variants hosi, hosin, Tabari âssen “the foot of a door on which it turns,” cf. Skt. īṣā- “pole or shafts of a carriage or plough,” Av. aêša “the two shafts,” Mod.Pers. xiš “plough(share),” Gk. oiax “handle of rudder, tiller, helm,” PIE base *ei-, *oi- “pole, thill.”