A type of firearms that uses a lever to cycle the action. Essentially, after it fires a bullet/shotshell, a mechanism around the grip is pushed and then pulled back to eject the casing and chamber a new round. Commonly used in the American Frontier era, it is therefore a normal fixture in cowboy or Old West-themed fiction. Arnold Schwarznegger's lever-action shotgun in Terminator 2 is a recent usage in a modern setting.
Basically, if a gun has what looks like a sword's handguard around where the character holds the gun (grip), it is very likely a lever-action gun. Since lever-action guns can either be a shotgun or a rifle, this tag covers both instances.