Receiver
Also: Action
The central housing of a rifle: it holds the bolt, locks the cartridge in place, and contains the firing mechanism. The rifle's frame of reference.
The receiver is the rifle’s frame of reference. The barrel threads into it; the bolt cycles inside it; the stock cradles it; the scope rail mounts to it. Receiver flex under firing is one of the most pernicious sources of dispersion, which is why match actions are made from precision-ground billet steel.
Custom action makers (Defiance, BAT, Stiller, ARC) have largely replaced converted Remington 700 receivers at the high end of precision rifle competition because the tolerances are tighter and consistency is better lot-to-lot.