During which leader's tenure did the Soviet Union's "Era of Stagnation" begin?

The "Era of Stagnation" or "Stagnation Period" or "Brezhnevian Stagnation" began during the rule of Soviet leader Leonid Brezhnev. Prior to Brezhnev, during Nikita Khrushchev's tenure there were relatively liberal reforms introduced (referred to as "Khrushchev Thaw"). Brezhnev rolled back several reforms implemented by Khrushchev, along with re-instituting Stalinist policies. It was during this roll back when industrial growth rates dropped and more emphasis placed on weapons and heavy industries, while consumer-based goods suffered.

Following the death of Brezhnev in 1982, modest reforms were introduced by the two short-term successors (Yuri Andropov and Constantin Chernenko). Further reforms were introduced by Mikhail Gorbachev, culminating in "Perestroika", a restructuring of the Soviet economy, but ended with the dissolution of the Soviet Union in 1991.

The term "Era of Stagnation" was coined by Mikhail Gorbachev to describe how he viewed the economic, political, and social policies of the period.