Kaiser is the German word for "emperor". Like the Bulgarian, Serbian and Russian Czar it is directly derived from the Roman emperors' title of Caesar, which in turn is derived from the personal name of a branch of the gens (clan) Julia, to which Gaius Julius Caesar, the forebear of the first imperial family, belonged. Although the British monarchs styled "Emperor of India" were also called "Kaisar-i-Hind" in Hindi and Urdu, this word, although ultimately sharing the same Latin origin, is derived from the Greek: Καῖσαρ (kaisar), not the German Kaiser.
In English, the term 'the Kaiser' is usually reserved for the emperors of the German Empire and the emperors of the Austrian Empire. During the First World War, anti-German sentiment was at its zenith; the term the Kaiser—especially as applied to Wilhelm II of Germany—thus gained considerable negative connotations in English-speaking countries.
The Holy Roman Emperors (962–1806) called themselves Kaiser, combining the imperial title with that of King of the Romans (assumed by the designated heir before the imperial coronation); they saw their rule as a continuation of that of the Roman Emperors and used the title derived from the title Caesar to reflect their supposed heritage.