Final answer:
Mikhail Gorbachev's tenure as leader of the Soviet Union was characterized by political and economic reform through his policies of perestroika and glasnost, ultimately contributing to the Soviet Union's collapse in 1991.
Step-by-step explanation:
The period of the Soviet Union under the leadership of Mikhail Gorbachev is best described as a period of political and economic reform. During his time in office, Gorbachev initiated significant reforms known as perestroika (economic restructuring) and glasnost (increased political openness). These reforms sought to modernize the Soviet system and make political life more open, while also restructuring the economy towards a market-like system. Unfortunately, these reforms ended up exposing deeper problems within the Soviet economy, which contributed to its eventual collapse in 1991.