Bastion was a codename given to a planet that served as a temporary capital for the Imperial Remnant. This involved setting up a short-term base of operations and moving the Council of Moffs, military resources, and personnel to this temporary location.
In the closing years of the Galactic Civil War, the capital was shifted from one planet to another, with each one being temporarily called "Bastion." These worlds were linked not only by their role as the ongoing meeting place for the Moff Council, but also by their function as the Empire's administrative center. They housed the state archives moved from Coruscant in either 7 or 10 ABY, along with the associated bureaucratic staff, and also served as the location of Bastion Military Control, which was the core structure of the Imperial Navy.
It's plausible that Orinda and Ord Cantrell may have been early Bastions. However, by 12 ABY, the New Republic didn't know the Imperial capital's location, leading Dal Konur, a renegade Jedi and former Alliance Special Forces commando, to try and find the planet. By 19 ABY, the main administrative body had moved to Sartinaynian, the capital of the Braxant sector. This location remained the capital of Imperial territory for over a century and eventually became known as Bastion itself.