An Etymological Dictionary of Astronomy and Astrophysics

Optics: A deviation in the propagation direction of a beam of light.
Tilt quantizes the average slope in both the X and Y directions of a → wavefront or phase profile across the pupil of an optical system.

Etymology (EN): M.E. tylten “to upset, tumble,” from tealt “unsteady” (cf. O.N. tyllast “to trip,” Swed. tulta “to waddle,” Norw. tylta “to walk on tip-toe,” M.Du. touteren “to swing”).

Etymology (PE): Gerâ, present stem of 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).