I agree with drunkenpoacher. The BATF assigned a DAO classification to the Glock trigger action when it was introduced. They basically had a menu with three options: single action, DA/SA. and DAO and DAO was the closest match. Glock was very happy to have them do so, because at the time of its introduction, many law enforcement agencies mandated DAO handgun designs. But I really think it would have been better to come up with a new category such as "striker-action" or "constant-action", or at the very least, qualify the DAO status by calling it a "pre-set DAO", which is what it in truth is.
Back in the revolver days, double action and single action had real meaning, and described very fundamentally different trigger actions with distinctly different pull weights and lengths of pull. The terms single action, double action, and double action only translated pretty well to auto-loading, hammer-fired pistols. Double action basically meant "trigger cocking", and in a double action pistol or revolver, so long as a live round was chambered or was in the next chamber of the cylinder, pulling the trigger accomplished everything necessary for ignition. Double action hammer-fired pistols also had second strike capability.
The term DAO has now been extended to striker-action pistols in which pulling the trigger alone will not result in ignition unless the striker has been "pre-set" by slide reciprocation, and do not have second-strike capability. These are very fundamental differences from the original meaning of double action. What is worse, the term DAO has now been applied to pistols in which the striker is virtually completely tensioned by slide reciprocation, such as the P320, whereas other striker-fired pistols with not very different trigger pull length or feel, like the Springfield Armory XD, are classified as single action. So the term DAO when applied to striker-action pistols has lost most of its predictive value in determining the nature of the trigger pull, and is now being applied to pistols with remarkably different trigger pull weights and lengths than the double action, hammer-fired pistols and revolvers that the term originated with.