The Byelorussian Soviet Socialist Republic (Byelorussian SSR or BSSR; Bielaruskaja Savieckaja Sacyjalistycnaja Respublika, Bielaruskaja SSR; Belorusskaya Sovetskaya Sotsialisticheskaya Respublika, Belorusskaya SSR), also referred to as Byelorussia, was one of fifteen constituent republics of the Soviet Union (USSR). To the west it bordered Poland. Within the USSR, it bordered the Lithuanian and Latvian SSRs to the north, the Russian SFSR to the east and the Ukrainian SSR to the south.