In firearms terminology, the Cylinder refers to the cylindrical, rotating part of 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...

 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. Each time the gun is cocked, the cylinder indexes

Indexing (motion)

Indexing in reference to motion is moving into a new position or location quickly and easily but also precisely. After a machine part has been indexed, its location is known to within a few hundredths of a millimeter , or often even to within a few thousandths of a millimeter , despite the fact...

 by one chamber (in six-shooters, 60°

Degree (angle)

A degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians...

).

Cylinders typically hold six cartridges (hence revolvers sometimes being referred to as "sixguns"), but some small frame revolvers only hold 5 cartridges, due to the smaller overall size of the gun and reduced space availability.