A combining form (also used adjectively) [
1913 Webster]
" 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]