[1] : The Collaborative International Dictionary of English v.0.48

ortho- \or"tho-\ ^or"th-o- pref. gr. 'orqo`s straight;
   akin to skr. =urdhva upright, v.rdh 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
see also:
normal meta- para- benzene nucleus benzene 

Ortho- >> O Words
Ortho-, definition of term: Ortho-
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