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. Chem.
a. 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- .
b. 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, esp. before a vowel, orth- .
[ orthós straight, upright, correct ]