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1.
The nuclear components that comprise the core of a nuclear weapon; as opposed to the non-nuclear components.

The combination of fissile uranium/plutonium, fusion fuel, high explosive lenses, tampers, and x-ray reflecting materials would be considered the 'physics package'. Other components of a weapon, such as guidance systems, triggers, and motors or engines would not.
I want you to open up this Tomahawk missile, so we can see what's gone wrong. But don't touch the physics package!
by dr_strangelove444 December 13, 2009
 
2.
n. (Military jargon) The essential workings of a nuclear or thermonuclear weapon. The "physics package" is designed by scientists at a weapons lab and comprises all the nuclear reactants used in the device, such as plutonium, lithium deuteride, etc. The military then decides how to deploy it (missiles, bombs, backpacks).
Military weaponeer: "The B-66A is an advanced, bunker-busting, anti-raghead weapon that is based on the Los Alamos E-1337 physics package. The guidance system uses a special neural network code to identify and deliver the device against brown-skinned persons of the Muslim persuasion..."

Dick Cheney: (Evil grin) "We're pleased with your rapid progress. The so-called "Ramadan" starts next month!"
by Carl Willis October 17, 2004