SDI stands for Strategic Defense Initiative. It was a US military plan that President Reagan started in 1983. The idea was to build special weapons — many of them on satellites in space, some using lasers — that could destroy another country’s missiles before those missiles reached America. Newspapers nicknamed it Star Wars, after the films, because the picture in people’s heads was space lasers shooting down rockets. Many scientists said it would not work, or would cost far too much, and it was never built as a finished shield in the sky. It still mattered in the 1980s, because the Soviet Union had to take it seriously in talks. If you meet SDI in a history book, it is that Reagan-era plan, not a general word for “defence” and not a later missile programme with a different name.