Short answers, pulled from the story.
It was the joint leadership of Russia between 2008 and 2012, during which Vladimir Putin served as prime minister under President Dmitry Medvedev after being constitutionally barred from a third consecutive presidential term.
The term is a political neologism, a portmanteau formed from "tandem," the interfix "-o-," and "-cracy," meaning rule.
Presidential power formally passed from Putin to Medvedev on the 7th of May 2008, a transfer that some commentators and analysts said was symbolic only.
Opinions differed sharply. In January 2009, Levada Center polling found that 32% of Russians believed Putin held the real power, 11% believed Medvedev did, and 50% believed both men held it equally. That same month, Putin's approval rating of 83% outpaced Medvedev's 75%.
Putin was re-elected president in the 2012 election and Medvedev became his prime minister, reversing the roles the two men had held since 2008.
Constitutional amendments passed in 2020 allowed Putin to seek a third consecutive presidential term, a bid he pursued in the 2024 election.