ortho-

from The Century Dictionary.

• noun An element in many words of Greek origin, its presence bringing in the sense of ‘straight,’ ‘upright,’ ‘right,’ ‘correct.’

from the GNU version of the Collaborative International Dictionary of English.

• prefix A combining form signifying straight, right, upright, correct, regular.
• prefix (Chem.) A combining form (also used adjectively)
• prefix (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. Cf. Normal.
• prefix (Organic Chem.) Connection with, or affinity to, one variety of isomerism, characteristic of the benzene compounds; -- contrasted with meta- or para-; ; hence, designating any substance showing such isomerism.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• prefix straight, right, proper
• prefix chemistry Having an extra proportion of water
• prefix chemistry in isomeric benzene derivatives, having the two substituents in adjacent positions (compare meta- and para-)
• prefix physics of any molecule of the form X2 in which the two nuclei have parallel spin