parahydrogen
/ˌparəˈhʌɪdrədʒ(ə)n/Chemistry
noun
The form of molecular hydrogen in which the two nuclei in the molecule have antiparallel spins (so that the spectrum exhibits no hyperfine structure) and which forms about 25 per cent of hydrogen under normal conditions.
Origin
1920s; earliest use found in Chemical Abstracts. From para- + hydrogen, after German Parawasserstoff.