Alert Fighters (AKA Alert Vipers) are those Vipers prepped for launch at a moment's notice. A warship will have a Combat Air Patrol already flying around the ship, but the rest of the pilots on watch will be on standby in or near their Vipers.
If any threat is spotted, the alert fighters will be launched immediately to join the CAP in engaging. In the event of a large scale battle, the alert fighters will launch as merely the first wave with other waves soon to follow them.