Single Action (SA)
A trigger that performs only one action — releasing an already-cocked hammer — giving a short, light pull.
In a single-action design, the trigger does just one thing: it releases a hammer that has already been cocked (manually, or by the slide cycling). Because the trigger isn't also cocking the hammer, the pull is short and light, which aids precision. The 1911 is the classic single-action pistol; it's carried 'cocked and locked' with the hammer back and thumb safety on.
The trade-off is that single-action guns require the hammer to be cocked to fire, and the light trigger demands disciplined safety habits and usually a manual safety. Single-action revolvers, by contrast, must be manually cocked for each shot. SA triggers are loved by target shooters for their crispness.