x1 is a thermonuclear device/atomic nuclear fusion bomb with primary core/fusing material/isotopes x2.
In practice, the classic hydrogen bomb operates via the well-timed detonation of well-placed nuclear-fission bomb charges which surround the main core od the bomb. Thus, all such hydrogen bombs are both fusion and fission bombs. This word refers to only, or at least emphasizes, the fusion bomb aspect; the fissile material which is being detonated and the related mechanisms thereof are not intended for the x2 slot of this word. See also: "ratmijypo'ijbama".