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 ).
a combining form used in the name of that acid in a given series of acids that contains the most water (orthoboric acid ). Compare meta-, pyro-.
a combining form used in the names of the salts of these acids: if the acid ends in -ic, the corresponding salt ends in -ate (orthoboric acid (H 3 BO 3 ) and potassium orthoborate (K 3 BO 3 )); if the acid ends in -ous, the corresponding salt ends in -ite (orthoantimonous acid (H 3 SbO 3 ) and potassium orthoantimonite (K 3 SbO 3 )).
Also, especially before a vowel, orth-.

< Greek, combining form of orthós straight, upright, correct

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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]

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Word Origin & History

a comb. element in forming scientific and technical words, from Gk. ortho-, stem of orthos "straight, true, correct, regular," from PIE *eredh- "high" (cf. Skt. urdhvah "high, lofty, steep," L. arduus "high, steep," O.Ir. ard "high").
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

ortho- or orth-

  1. Straight; upright; vertical: orthotropic.

  2. Correct; correction: orthopsychiatry.

  3. Hydrated form of an acid or of its salts: orthoboric acid.

  4. Diatomic molecules in which the nuclei have the same direction of spin: orthohydrogen.

  5. ortho-
    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.

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