A moon clip is a ring-shaped or star-shaped piece of metal

Metal

A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

 designed to hold multiple cartridges together as a unit, for simultaneous insertion and extraction from a revolver

Revolver

A revolver is a repeating firearm that has a cylinder containing multiple chambers and at least one barrel for firing. The first revolver ever made was built by Elisha Collier in 1818. The percussion cap revolver was invented by Samuel Colt in 1836. This weapon became known as the Colt Paterson...

 cylinder

Cylinder (firearms)

In firearms terminology, the Cylinder refers to the cylindrical, rotating part of a revolver containing multiple cartridge chambers. The cylinder revolves around a central axis in the revolver to bring each individual chamber into alignment with the barrel for firing...

. Moonclips may either hold an entire cylinder's worth of cartridges together, half a cylinder (half-moon clip), or just two neighboring cartridges.

Moon clips can be used either to chamber rimless cartridges in a double-action revolver (which would normally require rimmed

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...

 cartridges), or to chamber multiple rimmed cartridges simultaneously.