Caesar is the family name or adopted title of the early Roman emperors. 

Caesar was a title which came from the family name of Julius Caesar, who ruled Rome as a monarch without a crown from 49 to 44 B.C.

Octavian, Caesar's nephew and adopted son, took his uncle's name and also the title of Augustus. The next four Roman emperors all had some claim, by family or adoption, to the name of Caesar, which became so closely associated with the idea of the emperor that it was a kind of title.

In choosing the person to follow him as supreme ruler, the emperor would confer upon his heir the title Caesar. In the days of the Byzantine Empire, anyone chosen as ruler of a country under the Empire might be called Caesar. In the Russian language the title became czar. In German, Caesar was changed to kaiser.