pistol
A semi-automatic handgun with a magazine.
More technically, a pistol is a handgun whose chamber is integral with the barrel. This is opposed to a revolver, which has multiple chambers in a cylinder separate from the barrel. Single-shot handguns without a magazine, such as the Derringer or flintlock handguns, are also considered pistols.
In everyday language, the terms pistol and handgun are often used interchangeably. On Danbooru, a pistol is a specific type of handgun. Revolvers should be tagged as handguns, not pistols.
See also
This tag implicates handgun (learn more).