ortho-

American Heritage® Dictionary of the English Language, Fourth Edition

1. Straight; upright; vertical: orthotropous.
2. Perpendicular: orthorhombic.
3. Correct; correction: orthopsychiatry.
4. The most fully hydrated form of acid or of its salts: orthoboric acid.
5. Diatomic molecules in which the nuclei have the same spin direction: orthohydrogen.
6. Of or relating to one of three possible isomers of a benzene ring with two attached chemical groups in which the carbon atoms with attached groups are adjacent: ortho-dibromobenzene.

Century Dictionary and Cyclopedia

1. n. An element in many words of Greek origin, its presence bringing in the sense of ‘straight,’ ‘upright,’ ‘right,’ ‘correct.’ In chem., specifically — As a prefix of benzene derivatives it denotes a substitution of hydrogen atoms in the benzene ring which are adjacent to each other.

Wiktionary

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

GNU Webster's 1913

1. A combining form signifying straight, right, upright, correct, regular.
2. (Chem.) A combining form (also used adjectively)
3. (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.
4. (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.