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!