Ortho

[awr-thoh] /ˈɔr θoʊ/

adjective, Chemistry.
1.
pertaining to or occupying two adjacent positions in the benzene ring.
Compare 2 , 3 .
1.
a combining form occurring in loanwords from Greek, where it meant “straight,” “upright,” “right,” “correct” (orthodox) and on this model used in the formation of compound words (orthopedic).
2.
Chemistry.

combining form
1.
straight or upright: orthotropous
2.
perpendicular or at right angles: orthoclastic
3.
correct or right: orthodontics, orthodox, orthography, orthoptics
4.
(often in italics) denoting an organic compound containing a benzene ring with substituents attached to adjacent carbon atoms (the 1,2- positions): orthodinitrobenzene, o- Compare oarsman Compare oarsman
5.
denoting an oxyacid regarded as the highest hydrated form of the anhydride or a salt of such an acid: orthophosphoric acid See meta- (sense 6)
6.
denoting a diatomic substance in which the spins of the two atoms are parallel: orthohydrogen See para-1 (sense 8)

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-
pref.

American Orthopsychiatric Association