An Etymological Dictionary of Astronomy and Astrophysics

1. The angle between two lines or two planes.
   
2. An orbital element that defines the angle between the orbital plane of a solar system body (planet, comet, asteroid) and the plane of the ecliptic.
   
3. The angle between the equatorial and orbital planes of a planet.

Etymology (EN): Inclination, from M.E. inclinacioun, from L. inclination-, from inclinat(us) p.p. of inclinare, from → in- + clinare “to bend;” cf. Gk. klinein “to cause to slope, slant, incline;” Skt. sri- “to lean,” O.Pers. θray-; Av. sray- “to lean;” Mod.Pers. kil “bent, inclined, slant;” PIE base *klei- “to lean, incline” from which is also derived
P.Gmc. *khlinen (Ger. lehnen, E. lean).

Etymology (PE): Darkil, from prefix dar- “in” + kil “bent, crooked, inclined” [Dehxodâ], Mid.Pers. xwahl “bent, crooked” (Mod.Pers. dialectal/colloquial variants kowleh, cowleh, cowl, caval, xohl, xohal, hol), cf. Skt. kûla- “slope, declivity;” PIE base *klei-, as above.