thermonuclear UK /ˌθɜː.məʊˈnjuːk.liː.ə/US /ˌθɜɹ.moʊˈn(j)uk.li.əɹ/adjective 2noun 1forms 1 adjective2 •Of, or relating to the fusion of atomic nuclei at high temperatures.The first thermonuclear reaction was in a star; it was brought about by heat, as the description implies. — 1956 •Of, or relating to the use of atomic weapons based on such fusion, especially as distinguished from those based on fission.I can't believe you gave me, a ten-year-old boy, a thermonuclear device [...] — 2021 noun1 •A thermonuclear weapon.China detonated her first thermonuclear in June 1967. formsthermonuclears