What is a Revolver? The ATF Definition

Defined Under Federal Laws: 18 U.S.C., § 921(A)(29) and 27 CFR § 478.11:

The term “Revolver” means a projectile weapon of the pistol type, having a breechloading chambered cylinder so arranged that the cocking of the hammer or movement of the trigger rotates it and brings the next cartridge in line with the barrel for firing.

As for the terminology and nomenclature of revolvers, the first illustration shows the primary characteristics exhibited in the "Revolver" category.

The second illustration below further details the primary characteristics exhibited in the "Revolver" category. Since revolver configurations differ significantly, various models in this classification may exhibit any of the illustrated components in a revised configuration, but not necessarily all components shown will be incorporated in any one given design.

For more on terminology and nomenclature of rifles, please click:

Firearm Terminology & Nomenclature: Top Break Action

This is per the ATF website:

Firearms - Guides - Importation & Verification of Firearms - Gun Control Act Definition - Revolver