CHESSNOID

I knew we had Presidents and not Kings in the United States, but I was not aware that we had many Czars.  What is the definition of czar?  I always thought it was a Russian King or something like that.
Dictionary.com:
–noun

1.
an emperor or king.

2.
(often initial capital letter) the former emperor of Russia.

3.
an autocratic ruler or leader.

4.
any [...]