1. Prefix indicating that a benzene compound has two substituted groups in the 1,2 positions (i.e. on adjacent carbon atoms). The abbreviation o- is used; for example o-dichlorobenzene is 1,2-dichlorobenzene. Compare meta-; para-.
2. Prefix formerly used to indicate the most hydrated form of an acid. For example, phosphoric(V) acid, H3PO4, was called orthophosphoric acid to distinguish it from the lower metaphosphoric acid, HPO3 (which is actually (HPO3)n).
3. Prefix denoting the form of diatomic molecules in which nuclei have parallel spins, e.g. orthohydrogen (see hydrogen). Compare para-.