noun Physics, Chemistry.
the form of molecular hydrogen in which the nuclei of the two hydrogen atoms contained in the molecule have spins in the same direction.
Compare parahydrogen.

1925–30; ortho- + hydrogen

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World English Dictionary
orthohydrogen (ˌɔːθəʊˈhaɪdrədʒən)
chem Compare parahydrogen the form of molecular hydrogen, constituting about 75 per cent of the total at normal temperatures, in which the nuclei of the atoms spin in the same direction

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