noun: (astronautics) The structure, generally containing mechanical components and instruments but no engines or fuel tanks, connecting the top of a stage of a multistage rocket to the bottom of the stage above it.
noun: (military) The components of a multistage thermonuclear weapon which channel the radiation produced by the primary's fission explosion so as to trigger the implosion of the secondary and ignite a fusion explosion in the secondary's fusion fuel.
