Prefix
A combining form signifying straight, right, upright, correct, regular; as, orthodromy, orthodiagonal, orthodox, orthographic.
A combining form (also used adjectively)
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.
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.
Source: Webster's dictionaryFor cupolae and rotundae, they can be joined so that like faces (ortho-) or unlike faces (gyro-) meet. Source: Internet
Plant thiamine antagonists are heat-stable and occur as both the ortho- and para-hydroxyphenols. Source: Internet