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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).
  1. 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-.
  2. 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-.
Origin of ortho-
< Greek, combining form of orthós straight, upright, correct Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2018.
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British Dictionary definitions for ortho-


combining form
straight or upright: orthotropous
perpendicular or at right angles: orthoclastic
correct or right: orthodontics, orthodox, orthography, orthoptics
(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
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)
denoting a diatomic substance in which the spins of the two atoms are parallel: orthohydrogen See para-1 (sense 8)
Word Origin
from Greek orthos straight, right, upright
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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Word Origin and History for ortho-

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").

Online Etymology Dictionary, © 2010 Douglas Harper
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ortho- in Medicine

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.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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