An Etymological Dictionary of Astronomy and Astrophysics

Optics: A measure of the convergence or divergence of a pair of light rays, defined as the reciprocal of the distance between the point of focus and a reference plane.
Ophthalmology: The turning motion of the eyeballs toward or away from each other.

Etymology (EN): back formation from → convergence and → divergence, ultimately from L. vergere “to turn, bend, be inclined;” cognate with Pers. gardidan “to turn, to change,” → version.

Etymology (PE): Gerâyi, from gerâyidan “to incline toward; to intend; to make for.”
Gerâ may be a variant of Mod.Pers. kil “bent, inclined” (k/g and l/r interchanges),
from PIE base *klei- “to lean, incline,” cognate with L. clinare “to bend” (E. declination, inclination, etc.), Gk. klinein “to cause to slope, slant, incline,” Skt. sri- “to lean,” O.Pers. θray-, Av. sray- “to lean,” P.Gmc. *khlinen (Ger. lehnen, E. lean).