Source : Webster's Revised Unabridged Dictionary (1913)

Ortho- \Or"tho-\ [Gr. ? straight; akin to Skr. ?rdhva upright,
   vrdh to grow, to cause to grow.]
   1. A combining form signifying straight, right, upright,
      correct, regular; as, orthodromy, orthodiagonal, orthodox,
      orthographic.

   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.

   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.