1. (n.) A combining form signifying straight, right, upright, correct, regular; as, orthodromy, orthodiagonal, orthodox, orthographic.

2. (n.) A combining form (also used adjectively)

3. (n.) 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. (p. p.) 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.