An off-world base maintained by Stargate Command. There are, in fact, two versions of the Beta site:
1. Name given to the Alpha Site equivalent in an alternate reality that Daniel Jackson visited during the first year of the S.G.C.'s operation. There the Beta Site was a refuge from Apophis's invasion of Earth, though only a handful of people made it there.
2. Secondary off-world base established after Jonas Quinn's memory was probed by Anubis. Many Alpha Site troops evacuated to the Beta Site after Earth's primary off-world base was discovered and attacked by the forces of Anubis. There the Tok'ra and Jaffa rebels feuded over how the Goa'uld discovered the secret base.
There But For the Grace of God - Jackson is sent to an alternate reality, where Earth's best and brightest are being evacuated to the Beta Site under the looming threat of a Goa'uld invasion of Earth.
Death Knell - Survivors of the Alpha Site journey to the Beta Site and almost immediately are set against each other over suspicions of who leaked the coordinates of their destroyed base.