"Physics package" is a euphemism for the portion of a nuclear weapon that includes the actual explosive portion of the weapon: the detonator explosives, the fissile material, and (for fusion weapons) fusion fuel.
Many nuclear weapons are built in a modular fashion, with the same physics package being used for several different warhead applications. For example, the physics package of the B61 nuclear bomb is also used in the W80 ALCM warhead, W81 for the cancelled nuclear Standard SM-2 SAM, W84 GLCM warhead, and W85 Pershing II missile.
