A DSc is a very high university title in a science subject: Doctor of Science. In Britain and some other countries it is often above a PhD — given for a large body of important research already published, not for one student thesis. Universities also give a DSc as an honorary degree: a special honour to a famous scientist, even if that person did not study there or sit the usual exams. You write it after a name, like other degree letters. It is formal. Though actually, not every country uses DSc this way; in some places “Doctor of Science” is simply their name for a research doctorate, closer to a PhD. On a British invitation or CV, think “very high science honour,” not “the ordinary degree you get after a few years at university.” It is not a medical doctor.