The official name for modern-day Russia under the Communist Party from 1936 to 1991 (from 1918 to 1937, according to the Constitution of the RSFSR, called the Russian Socialist Federative Soviet Republic). Part of the USSR from 1922 to 1991.
How often have the words RSFSR and Russian Soviet Federative Socialist Republic occurred in a corpus of books? (source: Google Ngram Viewer )