The main island of the United Kingdom.
Questions tagged with britain should refer to historical states on the Great Britain island. The modern-day (since 1801) state United Kingdom of Great Britain and Ireland or (since 1922) United Kingdom of Great Britain and Northern Ireland should be tagged with united-kingdom. Do not use britain for former existing British Empire; mark questions with british-empire tag instead.
Questions related to previous countries on the Great Britain island can be also tagged with england or scotland. Questions related to ancient Britannia province of the Roman Empire should also be tagged with roman-empire.
Questions related to Ireland and Northern Ireland (Ulster) should be tagged with ireland.