With regard to
firearms, refers to a
trigger that performs one action, namely releasing the
hammer. A single action
gun must be cocked by some other mechanism. On a
revolver, this would be by thumbing back the hammer. On a auto-loading
pistol, the hammer is cocked by the recoil of the slide.
Compare this to the operation of a double action gun which can be cocked and fired by one pull of the trigger.