The Czechoslovak Socialist Republic (Czech/Slovak: Československá socialistická republika, ČSSR) was the official name of Czechoslovakia from 11 July 1960 until following the 1989 Velvet Revolution, when the name was changed on 23 April 1990 - it subsequently split into the Czech Republic and Slovakia. The CSSR was been regarded as a satellite state of the Soviet Union.
Following the coup d'état of February 1948, when the Communist Party of Czechoslovakia seized power with the backing of the Soviet Union, the country was declared a people's republic after the Ninth-of-May Constitution became effective. The traditional name Československá republika (Czechoslovak Republic) was changed on 11 July 1960 following implementation of the 1960 Constitution of Czechoslovakia as a symbol of the "final victory of socialism" in the country, and remained so until the Velvet Revolution in Czechoslovakia. Several other state symbols were changed in 1960.
More information on the English Wikipedia page [1]. The Czech page is [2] and the Slovak [3]