Definition: Single action (SA) refers to any firearm that must be manually cocked before each shot. In such guns, the trigger performs a single function (firing the gun), thus the "single" in the name.
Single action most often refers to handguns, but may also refer to long guns (rifles and shotguns) as well.
Most firearms of single action design are revolvers, but some semi-automatic pistols fit the bill, such as the Colt 1911, which must be manually cocked before the first shot may be fired, and is automatically cocked for successive shots. Such pistols are sometimes referred to as SAO, meaning "single action only."
Some semi-auto pistols are DA/SA (double action/single action), meaning that the first shot may be fired without manually cocking the hammer in a separate operation (double action), and each successive shot is then fired single action, because the automatic operation of the pistol (induced by the firing of a cartridge) cocks the gun to ready it for the next shot.
Examples: One of the best-known revolvers of all time is the Colt Single Action Army, the hammer of which must be cocked before each shot.