Citizen of the United Kingdom and Colonies

The term British subject has had a number of different legal meanings over time.

Formerly 'British subject' was used to denote de facto citizenship of the United Kingdom and the British Empire, and until 1949 was used to refer generally to any person born or naturalised in the United Kingdom or the British Empire, including the independent dominions such as Canada and Australia (but not including protectorates, e.g., the Princely States). The term had a more complex interpretation between 1949 and 1983 and the move to independence of many of the colonies, with subject status existing alongside citizenship of an individual country or colony.

Currently the term 'British subject' refers, in British nationality law, to a limited class of people defined by Part IV of the British Nationality Act 1981. Under that Act, two groups of people became "British subjects"; the first were people from the Republic of Ireland born before 1949 who already claimed subject status, and the second covered a number of people who had previously been considered "British subjects without citizenship", and were not considered citizens of any other country. This second group were predominantly residents of colonies which had become independent, but who had not become citizens of the new country. The status cannot be inherited, and is lost on the acquisition of any other citizenship; it will therefore cease to exist on the death of the last remaining subjects.

The term 'subject' is used rather than 'citizen' because in a monarchy the monarch is the source of authority in whose name all legal power in civil and military law is exercised. The people of a monarchy in former times were regarded as the monarch's subjects who were under certain obligations such as owing allegiance to, and thereby entitled to the protection of, the Crown.

...
Wikipedia