In nuclear weapons, boosted fission is a technique for injecting controlled quantities of tritium into a fission device core as it undergoes implosion. The presence of the tritium will increase the number of neutrons generated by the Primary nuclear weapon.
These neutrons can increase the fission yield, or can be directed into a Secondary fusion device. Variable-yield nuclear weapons are believed to use the amount of injected tritium to control the yield -- the more injected, the higher the yield. Timing of injection, however, is critical and has not always been successful in early tests by nations.
Tritium's relatively short half-life means that any nuclear weapon containing it must have its tritium supply replaced periodically.