The term belted magnum refers to any caliber cartridge, generally rifles, using a shell casing with a pronounced "belt" around its base that continues 2-4mm past the extractor groove

Rim (firearms)

A rim is an external flange that is machined, cast, molded, stamped or pressed around the bottom of a firearms cartridge. The rim may serve a number of purposes, the most common being as the place for the extractor to engage...

. This design originated with the British gunmaker Holland & Holland

Holland & Holland

Holland & Holland is a British gun-maker based in London, England. They offer hand-made sporting rifles and shotguns. H&H holds two Royal Warrants.-History:Holland & Holland was founded by Harris Holland in the year 1835....

 for the purpose of headspacing

Headspace

In firearms, headspace is the distance measured from the part of the chamber that stops forward motion of the cartridge to the face of the bolt. Used as a verb, headspace refers to the interference created between this part of the chamber and the feature of the cartridge that achieves the correct...

 certain of their more powerful cartridges. Especially the non-shouldered (non-"bottlenecked") magnum rifle cartridges could be pushed too far into the chamber and thus cause catastrophic failure of the gun when fired with excessive headspace; the addition of the belt to the casing prevented this over-insertion. An example of an American adaption of this practice is seen in cartridges like the .458 Winchester Magnum

.458 Winchester Magnum

The .458 Winchester Magnum is a belted, straight-taper cased, dangerous game rifle cartridge. It was introduced commercially in 1956 by Winchester and first chambered in the Winchester Model 70 African rifle. It was designed to compete against the .450 Nitro Express and the .470 Nitro Express...

, also a shoulderless heavy magnum.

Many subsequent cartridges of "magnum" nomenclature were based on the original .375 H&H

.375 H&H Magnum

The .375 Holland & Holland Magnum is a powerful rifle round and one of the best-known and most popular medium-bore cartridges in the world. The .375 H&H was only the second cartridge ever to feature a belt, now common among magnum rounds. A popular misconception is that the belt is for headspace,...

 cartridge, so over time the belt became something of a standardized attribute, expected as part of a 'magnum' cartridge. Many cartridge designs of the last century include this belt, but do not really require it.

In the last decade or so, there has been a trend toward beltless (rimless

Rim (firearms)

A rim is an external flange that is machined, cast, molded, stamped or pressed around the bottom of a firearms cartridge. The rim may serve a number of purposes, the most common being as the place for the extractor to engage...

) magnums, virtually all of which are heavily shouldered designs that obviate the original motivation for a belt.