The all-scale, often economical, change that occured in the 80's in the Soviet Union. Ultimately, perestroika collapsed the age of the Soviets. Now, the Soviet Union is called the CIS (Commonwealth of Independant States), aka : Russia.
"We live in a sad age...imperialism...totalitarianism...perestroika..."
-Revolver Ocelot in MGS2, after Snake survives the second round of torture
The economical programme pioneered by Mikhail Gorbachev in 1986, including social reform in the name of glasnost, allowing more economical and political freedoms to the Soviet populace. As populary mentioned, this programme was not the cause of the collapse of the already weakened Soviet Union but merely a catalyst for that inevitable event.
Perestroika was a step in the right direction, allowing the populace of the USSR to formulate their own devices for the future, thereby collapsing the USSR atop themselves, willingly and rightfully. Hail Mikhail Gorbachev!