noun

• (historical, uncountable) A program of political and economic reform carried out in the Soviet Union in the 1980s and early 1990s under the leadership of Mikhail Gorbachev.
• (historical, uncountable) The period of time in the Soviet Union during which these reforms were carried out and in effect.
• (countable, singularity theory) A situation where a small variation of parameters leads to a sudden change in properties.