A tsar was the male ruler — the emperor — of Russia in the days before the revolution of 1917. Nicholas II was the last one. A tsar held enormous power, ruling by inheritance rather than by election, and the word carries that weight of total, unquestioned authority. His wife or a woman ruling in her own right was a tsarina. You will also see it spelled czar, especially in older books and in American writing; tsar is the more common spelling in Britain today and closer to how the Russian sounds. Both are correct. There is a lively modern use worth knowing: in politics and news, a tsar is a person the government puts in charge of one problem with wide powers to fix it — a drugs tsar, a housing tsar. That use is informal, journalistic, and always a bit of an exaggeration, hinting that this person can simply order things done. Note that the word applies to Russia and some neighbouring lands, especially Bulgaria and Serbia in earlier centuries; do not use it for kings elsewhere.