Di is what British newspapers and magazines called Diana, Princess of Wales — the woman who married Prince Charles. They wrote it short and close, the way papers like to nick a famous first name, as if she were a pop star. Friends may have said Di in real life too, but the Di you meet in English reading is almost always that headline nickname. It is not a title, and it is not used for every woman named Diana. Though actually, after she died, many papers stopped using the cute short name and wrote Diana again, more carefully.