correct or right: orthodontics, orthodox, orthography, orthoptics
4.
(often in italics) denoting an organic compound containing a benzene ring with substituents attached to adjacent carbon atoms (the 1,2- positions): orthodinitrobenzene, o-Compare oarsmanCompare oarsman
5.
denoting an oxyacid regarded as the highest hydrated form of the anhydride or a salt of such an acid: orthophosphoric acid See meta- (sense 6)
6.
denoting a diatomic substance in which the spins of the two atoms are parallel: orthohydrogen See para-1 (sense 8)