A gun's action is its heart. This where the loading and firing mechanisms come together to form a working firearm and to dictate the way in which it functions.
Revolvers may be described as single action or double action. Other guns' actions may be semi-automatic, bolt action, lever action, pump (or slide) action, break action, or single shot. "Single shot" may be used to describe an action, and may itself be subdivided into more descriptive subcategories of actions, including (but not limited to) bolt action, falling block, Martini, rolling block, break action, tip-up, trapdoor, and Flobert.
Semi-automatic pistols may also be subcategorized as single action only (SAO), double action only (DAO), or single action/double action (SA/DA).