[orth-] or ortho- comb form [ME, fr. MF, straight, right, true, fr. L, fr. Gk, fr. orthos; akin to Skt urdhva high, upright] 1: straight: upright: vertical "orthograde"
2: perpendicular "orthorhombic"
3: correct: corrective "orthodontia"
4. a: hydrated or hydroxylated to the highest degree "orthophosphoric acid" b: involving substitution at or characterized by or having the relationship of two neighboring positions in the benzene ring "ortho-xylene"