before vowels orth-, word-forming element meaning "straight, upright, rectangular, regular; true, correct, proper," now mostly in scientific and technical compounds, from Greek ortho-, stem of orthos "straight, true, correct, regular," from PIE *eredh- "high" (cf. Sanskrit urdhvah "high, lofty, steep," Latin arduus "high, steep," Old Irish ard "high").
ortho- or orth-
Straight; upright; vertical: orthotropic.
Correct; correction: orthopsychiatry.
Hydrated form of an acid or of its salts: orthoboric acid.
Diatomic molecules in which the nuclei have the same direction of spin: orthohydrogen.
Abbr. o- Of or relating to one of three possible isomers of a benzene ring with adjacent carbon atoms having attached chemical groups. Usually used in italic: ortho-dibromobenzene.