An Etymological Dictionary of Astronomy and Astrophysics

Level and horizontal, without any slope; even and smooth, without any bumps or hollows.
See also:
→ flat manifold, → flat Universe, → flat-field, → flatness problem.

Etymology (EN): Flat, from O.N. flatr, from P.Gmc. *flataz (cf. O.H.G. flaz “flat, level,” O.E. flet, O.H.G. flezzi “floor”), perhaps from PIE *pla- (cf. Gk. platys “broad, flat;” Av. pərətu- “broad, wide;” Skt. prthu- “broad, wide, large”).

Etymology (PE): Taxt “flat;” Mid.Pers. taxtag “tablet, plank, (chess)board.”