An adjective for the kind of nuclear reaction that needs enormous heat to start — and for the weapons and machines built around it. In this reaction, very light atoms (usually forms of hydrogen) are squeezed and heated until they join together into one bigger atom, releasing a huge amount of energy. This is what happens inside the sun, and it is what makes a hydrogen bomb far more powerful than the first atomic bombs. Almost always used before a noun: a thermonuclear weapon, a thermonuclear war, a thermonuclear reaction, thermonuclear fusion. Because of the bomb, the word carries a heavy Cold War feeling — you meet it in news reports, history books, and politics much more than in ordinary science talk. English speakers also stretch it as an exaggeration for anything extreme: 'a thermonuclear argument' means a fight that blew up completely and destroyed the relationship. That use is informal and a little dramatic. One warning about definitions you may read: joining atoms together is called fusion, and splitting them apart is called fission. Thermonuclear reactions are the joining kind, so a description that says 'splitting of atoms' is loose — though real hydrogen bombs do use a small splitting bomb first, as the match that lights the heat.