ortho-

WordReference Random House Learner's Dictionary of American English © 2026

ortho-, prefix. 

1. ortho- comes from Greek, where it has the meaning "straight, upright, right, correct '':
   ortho- + graph → orthography (= correct writing);
   ortho- + dontics → orthodontics (= dentistry dealing with straightening teeth);
   ortho- + pedic → orthopedic (= correction of improper bone structure from childhood).

Collins Concise English Dictionary © HarperCollins Publishers::

ortho-, (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) denoting an organic compound containing a benzene ring with substituents attached to adjacent carbon atoms (the 1,2- positions)
   Compare meta-4
   Compare para-16
5. denoting an oxyacid regarded as the highest hydrated form of the anhydride or a salt of such an acid: orthophosphoric acid
   
6. denoting a diatomic substance in which the spins of the two atoms are parallel: orthohydrogen
   

Etymology: from Greek orthos straight, right, upright

WordReference Random House Unabridged Dictionary of American English © 2026

or•tho  (ôr′thō), adj. [Chem.]

1. Chemistry pertaining to or occupying two adjacent positions in the benzene ring. Cf. meta ², para ³.

• independent use of ortho- 1875–80

ortho-, 

1. 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).
2. Chemistry
   • a combining form used in the name of that acid in a given series of acids that contains the most water (orthoboric acid). Cf. meta-, pyro-. 
   • Chemistry 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,[esp. before a vowel,] orth-. 
• Greek, combining form of orthós straight, upright, correct