Very good responses to a fundamental question. Belted is a physical characteristic regardless of name -- speaks for itself. The term "Magnum" is merely a marketing ploy on the part of the manufacturer naming their cartridge.
Skeeter, that is a long list, and I agree with two exceptions. I believe that though the 7mmSTW and 458 Lott are indeed a belted cartridges; however, neither has been "knighted" with the title of Magnum. I checked all the literature I could find. A Smallbore and a Bigbore, yes. Magnums, no.
I wouldn't mention it other than I'm zeroing in acquiring on "capable" rifles that are NOT magnums: 416 Rigby, 458 Lott, 338 Lapua, 7mmSTW, etc. Wouldn't want to be accused of magnumitis.
EKM