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or orth-
1. Straight; upright; vertical: orthotropous.
2. Perpendicular: orthorhombic.
3. Correct; correction: orthopsychiatry.
4. The most fully hydrated form of an acid or of its salts: orthoboric acid.
5. Of or relating to one of three possible isomers of a benzene ring with two attached chemical groups in which the carbon atoms with attached groups are adjacent: ortho-dibromobenzene.

[Middle English, from Old French, from Latin, from Greek, from orthos, straight, correct, right.]


or before a vowel


combining form
1. straight or upright: orthotropous.
2. perpendicular or at right angles: orthoclastic.
3. correct or right: orthodontics; orthodox; orthography; orthoptics.
4. (Chemistry) (often in italics) denoting an organic compound containing a benzene ring with substituents attached to adjacent carbon atoms (the 1,2- positions): orthodinitrobenzene. Abbreviation: o- Compare meta-4 Compare para-16
5. (Chemistry) denoting an oxyacid regarded as the highest hydrated form of the anhydride or a salt of such an acid: orthophosphoric acid. See meta-6
6. (Chemistry) denoting a diatomic substance in which the spins of the two atoms are parallel: orthohydrogen. See para-18
[from Greek orthos straight, right, upright]


(ˈɔr θoʊ)

pertaining to or occupying two adjacent positions in the benzene ring. Compare meta, para 2.
[1875–80; independent use of ortho-]


1. a combining form meaning “straight,” “upright,” “right,” “correct”: orthodontics; orthopedic.
a. a combining form used in the name of the most hydrated acid in a given series: orthoboric acid. Compare meta- (def. 2a), pyro- (def. 2a).
b. a combining form used in the names of benzene derivatives in which the substituting group occupies the ortho position in the benzene ring.
Also, esp. before a vowel, orth-.
[< Greek, comb. form of orthós straight, upright, correct]