ortho- Also o-.
Straight; correct; upright.
[Greek orthos, straight, right.]
Orthodox literally means having the correct opinion (Greek doxa, opinion); orthoepy (Greek epos, epe-, word) is the correct or accepted pronunciation of words, or the study of it; orthodontics (Greek odous, odont-, tooth) is the treatment of irregularities in the teeth and jaws; something orthogonal (Greek gōnia, angle) involves right angles.
In chemistry, the form indicates substitution at two adjacent carbon atoms in a benzene ring (orthodichlorobenzene, ortho-aminoanisole), as opposed to the other two possible positions, meta- and para-1. It is often abbreviated to o-: o-tolidine, o-hydroxybenzaldehyde.
The form can also denote a compound in the highest state of hydration, containing the maximum number of hydroxyl groups: an example is an ortho acid such as orthosilicic acid. Such acids are often unstable and lose water to form compounds designated by meta-, as orthophosphoric acid becomes metaphosphoric acid on heating.