Single Action
A trigger system where the trigger only releases the hammer; the hammer must be cocked separately.
Also: SA
On a single-action (SA) firearm, the trigger does one thing: it releases the cocked hammer. The hammer must already be at full cock — typically by manually pulling it back, or by the action cycling on a semi-auto. SA triggers are very light and crisp because they don’t have to do the work of cocking the hammer.