The way Joseph Stalin ran the Soviet Union, treated as a system with a name. The pieces people usually mean: one man and a small circle around him decide everything; only one party is allowed and no one may argue with it; the state owns the farms and factories and pushes industry hard and fast, whatever it costs people; and fear does the work of persuasion — secret police, informers, arrests in the night, prison camps, show trials where the accused confess to things they did not do, and mass killing. Millions died, from the camps, from the shootings, and from famine caused by state policy.

The word also gets used for later governments run the same way, even far from Russia, and for movements that took Stalin as their model.

Outside history writing, people use it loosely as an insult for anyone who crushes disagreement — a boss, a school principal, a committee chair. That use is very common and very stretched: a strict manager is not running labour camps. Notice too that the word was mostly made by people criticising Stalin. Stalin's own side did not go around saying 'our Stalinism'; they said Marxism-Leninism. So the word arrives already carrying a judgement, which is worth knowing before you use it in writing.