Pegasus Alpha Site
Any one of a number of backup outposts in the Pegasus Galaxy, identified and staffed by the Atlantis expedition in the event that Atlantis is compromised. Plans to create an Alpha Site in Pegasus were in the works over a long-term period (as the Milky Way Galaxy Alpha Site has been a success), but efforts were redoubled following the discovery that three Wraith hive ships and their support vessels were en route to Atlantis.
Sergeant Bates and more than half a dozen men were sent to determine whether or not the initially selected site was viable, but while there the Wraith dropped a device near the Stargate. That they Knew about the world made the location useless. Bates believed Teyla Emmagan, who had recently tapped into the Wraith telepathic link, was responsible for giving away the location.
Eventually, after a great deal of searching, another world was selected.
The Gift - The selected Alpha Site is discovered by the Wraith, forcing Bates and his security team to retreat.
The Siege, Part 1 - Dr. Weir, Major Sheppard and Sergeant Bates search the Atlantis database for another viable Alpha Site.
The Siege, Part 2 - Colonel Everett, arriving from Earth through the Stargate, orders Major Sheppard to retrieve the officers and scientists stationed at the Alpha Site. There is no longer a need for the base, as his men will succeed in defending Atlantis.
Condemned - The team must dial one of the back-up planets for the Alpha Site when they make their escape from Olesia.