A revolver is a multishot firearm, usually a handgun, in which the rounds are held in a revolving cylinder that rotates to fire them through a single barrel.
Revolver-type weapons are part of the long development of making better multi-shot weapons. They were partly an attempt to improve on pepper-box type weapons, which used a revolving cylinder with one set of firing mechanisms, but which had multiple barrels as well. Firing through a single barrel saved the expense and weight of having the multiple barrels of the Pepper-box.