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.