An Etymological Dictionary of Astronomy and Astrophysics

1. (n.) a flat or level surface.
   

2. (adj.) Of or pertaining to planes or plane figures.

Etymology (EN): 1) From L. plantum “flat surface,” noun use of adj. planus “flat, level, plain.”
2) From L. planus, as above.

Etymology (PE): 1) Hâmon, variant of hâmun “plain, level ground;” Mid.Pers. hâmôn “level, flat;” Proto-Iranian *hāma-van-, from
*hāma- “same, equally, even; together, with” (Mod.Pers./Mid.Pers. ham-; O.Pers./Av. ham-; cf. Skt. sam-; also O.Pers./Av. hama- “one and the same;” Skt. sama-; Gk. homos-;
originally identical with PIE numeral *sam- “one,” from som-. The Av. ham- appears in various forms: han- (before gutturals, palatals, dentals) and also hem-, hen-) + -van- suffix.
2) Taxt “flat;” Mid.Pers. taxtag “tablet, plank, (chess)board.”