The purpose of this thread is so people can share their knowledge of older names and other terminology, to help avoid anachronisms when writing about a particular period. I'll start with some major ones:
Political right and left: These terms were invented during the French Revolution--in the national assembly at the time, the more conservative representatives sat on the King's right and the more radical ones on his left. If writing about a period before the French Revolution, you therefore cannot use terms like left and right.
The Byzantine Empire: The name 'Byzantine' was not coined until 1557, more than a century after the empire died, so obviously no-one at the time called it that. The people of the Empire themselves considered themselves Romans and used that name. Everyone else referred to it as "the Eastern Empire" or "the Greek Empire"--in opposition to "The Latin Empire", which had been the Western Roman Empire.
Britain: The term "United Kingdom" did not come about until 1801, when the Kingdom of Great Britain was added to the Kingdom of Ireland to form the United Kingdom of Great Britain and Ireland. Using the phrase "UK" as a term to describe the country was rare until the mid-twentieth century. Prior to that most people said either "Britain", or often "England" was (incorrectly, and usually by the establishment) considered synonymous with Britain or the UK. England and Scotland were united into Great Britain in 1707 by the Act of Union, and no longer exist as legal entities after that date: there is no 'Queen of England' or 'King of England'. However the Union Jack dates from 1603 or so, when James VI of Scotland became King James I of England in personal union. James wanted to combine the kingdoms politically (including merging the parliaments) but there was opposition at the time and it would only be done 100 years later. However his political efforts did mean that names like Monarch of All Great Britain were bandied about at the time, and for about 200 years the government sometimes insisted on using the politically correct terms "North Britain and South Britain" instead of Scotland and England--of course none of the ordinary people paid any attention.
Hung parliaments: This is now a common term in British politics but only dates from the 1970s; previously phrases like 'balanced parliament' or 'no overall majority' were used.
Democracy: In the eighteenth and early nineteenth centuries the word was considered to mean "mob rule" by most civilised society, and only radicals (like Andrew Jackson or the Chartists) would openly call themselves Democrats. It would not be used by any sort of establishment political party outside the United States.