The Borusa Interregnum was the name given to the short period of Gallifreyan history between the presidencies of Niroc and Flavia in which the Council of Administration, led by Borusa, was in power whilst the corrupt High Council was overthrown and new elections were held.
After the Sixth and Eighth Doctors exposed the involvement of the Celestial Intervention Agency, the High Council and Lord President Niroc in the Ravolox affair during the Sixth Doctor's trial, the Council all met in the Panopticon and formed lots of different minority groups to take over and reform the High Council. The Shobogans also planned a revolt.
To solve this anarchy, the Eighth Doctor persuaded Rassilon to release Borusa from his immortality that had been gifted to him during the Death Zone affair so that he could lead Gallifrey until new elections were organised to oust the Niroc and the other corrupt Councillors.
After new elections were held, Flavia was elected President to begin a long and successful reign whilst Borusa returned to Rassilon. (PROSE: The Eight Doctors)