ORTHO-

a combining form (also used adjectively).

2. ortho- ·- a combining form signifying straight, right, upright, correct, regular; as, orthodromy, orthodiagonal, orthodox, orthographic.

3. ortho- ·- the one of several acids of the same element (as the phosphoric acids), which actually occurs with the greatest number of hydroxyl groups; as, orthophosphoric acid. ·cf. normal.

4. ortho- ·- connection with, or affinity to, one variety of isomerism, characteristic of the benzene compounds;

— contrasted with meta- or para-; as, the ortho position; hence, designating any substance showing such isomerism; as, an ortho compound.