\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.