The secret police of East Germany, from 1950 until the country ended in 1990. Their work was not mainly arrests in the street; it was watching — opening letters, listening to phone calls, and above all persuading ordinary people to report on their neighbours, workmates, even their husbands or wives. By the end there were files on millions of citizens, and after 1990 people were allowed to read their own file and often found out who had informed on them. That history is why the word matters in English today. Outside German history books it is used as a comparison, and always a hostile one: to call a government agency, a company, or a school 'Stasi-like' is to say it spies on the people it is supposed to serve. It is a proper noun, so it takes a capital S and normally 'the' — 'the Stasi'. It is treated as a group, so it usually takes a plural verb in British English ('the Stasi were watching him'). One officer is 'a Stasi officer', not 'a Stasi'.