Ortho-

Ortho- Or"tho- ([^o]r"th[-o]-) pref. [Gr. 'orqo`s straight; akin to Skr. [=u]rdhva upright, v[.r]dh to grow, to cause to grow.] [1913 Webster] 1. A combining form signifying straight, right, upright, correct, regular; as, orthodromy, orthodiagonal, orthodox, orthographic. [1913 Webster]

2. (Chem.) A combining form (also used adjectively), designating: (a) (Inorganic Chem.) 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}. (b) (Organic Chem.) 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. [1913 Webster]

Note: In the graphic representation of the benzene nucleus (see {Benzene nucleus}, under {Benzene}), provisionally adopted, any substance exhibiting double substitution in adjacent and contiguous carbon atoms, as 1 & 2, 3 & 4, 4 & 5, etc., is designated by ortho-; as, orthoxylene; any substance exhibiting substitution of two carbon atoms with one intervening, as 1 & 3, 2 & 4, 3 & 5, 4 & 6, etc., by meta-; as, resorcin or metaxylene; any substance exhibiting substitution in opposite parts, as 1 & 4, 2 & 5, 3 & 6, by para-; as, hydroquinone or paraxylene. [1913 Webster]