Magnum Action
A rifle action lengthened and widened to feed belted and large magnum cartridges that do not fit a standard long action.
A magnum action is built with both extra length and extra width to handle the largest cartridges, including the belted rounds and big cases that overflow a standard receiver. The bolt face is opened up to grip a wider case head, and the magazine box is sized so the long, fat cartridges feed without binding, which is why a true magnum chambering usually rides in a dedicated receiver.
This is the next step up from a long action, reserved for cartridges that need more room than the .30-06 envelope provides. A classic belted magnum such as the .375 H&H is the reason this larger action exists, and many modern big-bore rounds inherit those dimensions.