A semi-automatic pistol (sometimes automatic pistol) is a handgun that automatically ejects and loads cartridges in its chamber after every shot fired. Only one round of ammunition is fired each time the trigger is pulled, as the pistol's fire control group disconnects the trigger mechanism from the firing pin/striker until the trigger has been released and reset.
Additional terms sometimes used as synonyms for a semi-automatic pistol are: self-loading pistol, autopistol, autoloading pistol, and automatic pistol.
A semi-automatic pistol recycles part of the energy released by the propellant combustion to move its bolt, which is usually housed inside the slide. After a round of ammunition is fired, the spent cartridge casing is extracted and ejected as the slide/bolt moves rearwards under recoil, the hammer/striker is cocked by the slide/bolt movement, and a new round from the magazine is pushed into the chamber when the slide/bolt returns forward under spring tension. This sets up the following shot, which is fired as soon as the trigger is pulled again. Most pistols use a short recoil operation to perform this, but some pistols use simple blowback or gas operation mechanisms.
Most types of semi-automatic pistols rely on a removable box magazine to provide ammunition, which is usually inserted into the grip. However, some pistols are based on receiver-style designs similar to existing semi-automatic rifles, and thus have the magazine inserted separately from the grip.
The language surrounding "automatic", "semi-automatic", "self-loading", etc., often causes confusion due to differences in technical usage between different countries and differences in popular usage. For example, the term "automatic pistol" technically refers to a fully automatic machine pistol, which is capable of continuously firing multiple rounds with a single pull of the trigger, although in popular American usage it is also used as a synonym for any self-loading pistol, the vast majority of which are semi-automatic.