What characterizes a firearm where every round fires double action with the hammer at rest?

A firearm where every round fires double action with the hammer at rest is characterized as double action only. In this type of mechanism, the trigger pull both cocks the hammer and releases it to fire the round, allowing for repeated firing without the need for the shooter to manually cock the hammer beforehand. This design is intended to simplify operation, making it easier and faster to discharge the firearm, especially in defensive situations where immediate action may be necessary.

Single-action only firearms require the hammer to be manually cocked before each shot, meaning the shooter must engage in an additional action which can slow down response time. A double/single action designation refers to a firearm that can operate in both modes; it can fire double action for the first shot and then single action for subsequent shots if the hammer is cocked, which is not applicable to this question's context. Revolvers are a type of firearm that can include both double action only and double/single action mechanisms but do not specifically define the operation of every round. Therefore, the defining characteristic of the firearm in question aligns solely with the double action only classification.