What is "ortho-"

Ortho- \Or"tho-\ ([^o]r"th[-o]-) pref. [Gr. 'orqo`s straight; akin to Skr. [=u]rdhva upright, v[.r]dh 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:

   1. (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.

   2. (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.

before vowels orth-, word-forming element meaning "straight, upright, rectangular, regular; true, correct, proper," now mostly in scientific and technical compounds, from Greek ortho-, stem of orthos "straight, true, correct, regular," from PIE *eredh- "high" (cognates: Sanskrit urdhvah "high, lofty, steep," Latin arduus "high, steep," Old Irish ard "high").

pre. 1 straight, right, proper 2 (context chemistry English) Having an extra proportion of water 3 (context chemistry English) in isomeric benzene derivatives, having the two substituents in adjacent positions (compare meta- and para-) 4 (context physics English) of any molecule of the form X₂ in which the two nuclei have parallel spin