In-universe, the correct spelling appears to be stormtroopers (with no capitals, no space), however there are a very considerable number of posts; both questions and answers, that refer to them as;
- Stormtroopers (Capital S)
- Storm troopers (with a space)
- Storm Troopers (Capital S, Capital T and a space)
As we're doing with lightsabers, should we systematically change all of these posts to the correct spelling?