A stormtrooper was a German soldier in a special assault unit — the men sent in first, fast and hard, to break through the enemy line. The idea started in the First World War and the word carried on into the Second World War. Because of that history, it is a cold word: if you call a real person a stormtrooper, you are saying they are a brutal soldier who follows orders without thinking. It is not a compliment and never a neutral job title.

There is a second, closely linked use that history books use a lot. In 1920s and 30s Germany, the Nazi party had its own violent street gang, the SA, and its members were called stormtroopers in English too (in German, Sturmabteilung). These were not army soldiers but party thugs who beat up opponents at meetings. So when you meet the word in a book about how Hitler came to power, it usually means these men, not wartime soldiers.

Now the honest complication, and it is a big one for your generation: most people today meet this word through Star Wars, where stormtroopers are the white-armoured soldiers of the Empire. That was a deliberate borrowing — the films took the name because of what it meant. In everyday conversation, especially among young people, the Star Wars meaning is the one that comes to mind first, and you will see the word joked about in that way constantly. But in a history class or a news article, the older meaning is the live one, and using it loosely there would sound careless.