In general, to be a national is to be a member of a state. Nationality is acquired by birth or adoption, marriage, or descent (the specifics vary from country to country). Having a nationality is crucial for receiving full recognition under international law. Indeed, Article 15 of the Universal Declaration of Human Rights declares that “Everyone has the right to a nationality” and “No one shall be arbitrarily deprived of his nationality nor denied the right to change his nationality” but is silent on citizenship. Citizenship is a narrower concept: it is a specific legal relationship between a state and a person. It gives that person certain rights and responsibilities. It does not have to accompany nationality. In some Latin American countries, for example, such as Mexico, a person acquires nationality at birth but receives citizenship only upon turning 18: Mexican children, therefore, are nationals but not citizens.
Similarly, not all American nationals are also American citizens. People born in the “outlying possessions of the United States” can get an American passport and live and work in the United States, but cannot vote or hold elected office. In the past, these “outlying possessions” included Guam, Puerto Rico and the U.S. Virgin Islands, but in the 20th century Congress gradually extended citizenship to their inhabitants. Today, only American Samoa and Swains Island stand apart: the latter is a tiny atoll in the Pacific ocean, barely more than five meters above sea level, which, in 2010, had a population of just 17.
In Britain, thanks to the legacy of colonialism, the situation is even more complicated. There are six types of British nationality: British citizens, British subjects, British overseas citizens, British overseas territories citizens, British overseas nationals, or British protected persons. Sometimes it is possible to switch categories: for instance, before the British handed Hong Kong over to the Chinese on 1st July 1997, some British overseas territories citizens registered as British overseas nationals. These overseas nationals hold British passports and can receive protection from British diplomats, but they do not have the automatic right to live or work in Britain. So in Britain, there are several types of citizenship and some nationals who are not citizens at all. The targets of Mrs May’s ire are likely to have good company in not fully grasping the meaning of the word “citizenship”.