World English Dictionary
ortho- or (before a vowel) orth-
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) Compare meta- Compare para- o- denoting an organic compound containing a benzene ring with substituents attached to adjacent carbon atoms (the 1,2- positions): orthodinitrobenzene
5.  See meta- denoting an oxyacid regarded as the highest hydrated form of the anhydride or a salt of such an acid: orthophosphoric acid
6.  See para- denoting a diatomic substance in which the spins of the two atoms are parallel: orthohydrogen
[from Greek orthos straight, right, upright]
orth- or (before a vowel) orth-
combining form
[from Greek orthos straight, right, upright]

Collins English Dictionary - Complete & Unabridged 10th Edition
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