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]