British refers to a person being a citizen of the United Kingdom. It is not usually used as an expression of nationality by British citizens, as many will prefer to use the expressions "English", "Scottish", or "Welsh", referring to which constituent country of the UK they self-identify with. An exception to this is its use in Northern Ireland by Unionists to show their support for the continued inclusion of Northern Ireland in the United Kingdom.
Historically, "British" was also the name given to the largest empire the world has ever seen, administered in the United Kingdom in the name of the British monarch. British colonies were established all over the world, covering roughly a quarter of the world's land surface area in 1921, even after losing much of its North American territory after the American Revolutionary War. The large empire was possible because Britain "ruled the seas" through the stong domination of the Royal Navy, and used this to ensure free trading for British interests. With up to a quarter of the world as British subjects, the Empire was also the world's foremost military power. For many centuries, populations lived under a period of relative peace and prosperity that was the Pax Brittannica, though from a modern perspective, the methods used to control and ensure this superiority on occasion were on occasion particularly vicious.
The empire as such no longer exists, though its legacy can still be appreciated today, be it through the popularity of traditional British sports such as soccer, cricket, rugby and golf; or from the status of English as the current global lingua franca; or from the use of the Westminster system of government and English common law around the world.
The Empire's successor is the Commonwealth of Nations, which still features the British monarch Queen Elizabeth II as the Head of the Commonwealth. The commonwealth consists of the UK, Commonwealth Realms which still have the British monarch as Head of State, and republics.
Recently, Australians voted in a referendum to retain the British monarch as their Head of State, rather than declare Australia a republic.