Double action

Double action or Double-action refers to one of two systems in firearms where the trigger both cocks and releases the hammer
Double-action only (DAO) firearms trigger: The trigger both cocks and releases the striker. There is no single-action function and the striker will return to its decocked position after each shot.  
Double Action Kellerman (DAK): A varant of traditional double-action used on certain SIG Sauer semi-automatic pistols.  DAK triggers have a long stroke with  pull.  However, if a user shooting under stress short-strokes the trigger by only releasing it halfway, the trigger will reset, but with a  pull.  This temporary increased trigger pull is intended to prevent negligent discharges.
Traditional double-action – or double-action/single-action (DA/SA) – firearms trigger: The trigger combines the features of both a double-action only and a single-action only trigger. The firing mechanism automatically cocks the striker after the gun is fired. This mechanism will cock and release the striker when the striker is in the down position, but on each subsequent shot, the trigger will function as a single-action when using a semi-automatic. With a revolver, the hammer can be cocked first (single action), or the trigger can be pulled and it will cock and release the hammer (double action). Once the gun has fired, the hammer stays in the decocked position until the hammer is re-cocked (single action), or the trigger is pulled again (double action).