[ awr-thoh ]
/ ˈɔr θoʊ /
Save This Word!

Definition of ortho

adjective Chemistry.
pertaining to or occupying two adjacent positions in the benzene ring.Compare meta3, para3.
Smoothly step over to these common grammar mistakes that trip many people up. Good luck!
Question 1 of 7
Fill in the blank: I can’t figure out _____ gave me this gift.

Origin of ortho

First recorded in 1875–80; independent use of ortho-

Other definitions for ortho (2 of 2)


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 (H3BO3) and potassium orthoborate (K3BO3)); if the acid ends in -ous, the corresponding salt ends in -ite (orthoantimonous acid (H3SbO3) and potassium orthoantimonite (K3SbO3)).
Also especially before a vowel, orth-.

Origin of ortho-

<Greek, combining form of orthós straight, upright, correct
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2023

How to use ortho in a sentence

British Dictionary definitions for ortho


before a vowel orth-

combining form
straight or uprightorthotropous
perpendicular or at right anglesorthoclastic
correct or rightorthodontics; 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 Abbreviation: o- Compare oarsman Compare oarsman
denoting an oxyacid regarded as the highest hydrated form of the anhydride or a salt of such an acidorthophosphoric acid See meta- (def. 6)
denoting a diatomic substance in which the spins of the two atoms are parallelorthohydrogen See para- 1 (def. 8)

Word Origin for ortho-

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