Kaiser is the German title meaning Emperor.
In German, the word is also used in a generic sense equivalent to the English emperor. For instance, German-speaking historians would refer to an emperor of China as a Chinesischer Kaiser. Cognate, nearly homophone titles, are used in the same ways in Germanic languages or those (mainly Baltic and Slavonic) who derived the term from German, for example:
Hebrew also uses the same word ("Keisar", Heb. קיסר) , though in this case the term did not go through German but came directly from Latin in the Roman period itself.
In contrast, most Romance and tributary vocabularies, including English, derive their terms for emperor from the Latin imperator.
The rulers of the Austro-Hungarian Empire (1806 - 1918), from the Habsburg dynasty that had provided all Holy Roman Emperors (though formally still elected) since 1440, again used the title Kaiser.
In English and most other foreign usage, however, the untranslated title is mainly associated with the emperors of the unified German Empire (1871 - 1918) (the "Second Reich") which chancellor Bismarck had welded skilfully from two federations covering most of the many principalities (mainly petty, known as Kleinstaaterei) that had constituted Germany, the core of the former Holy Roman Empire. The term is particularly associated in English with the last Kaiser, Wilhelm II.
There were three Kaisers of the German Empire. All belonged to the Hohenzollern dynasty, which had already ruled much of Germany ruled as kings of (originally 'in') Prussia, militarily the only great power among the German principalities, before ascending the brand new "German" imperial throne. These three Prussian Kaisers were:
Kejser | Kaiser | Imperiestro | Káiser | Keiser | Empereur | 皇帝 | კაიზერი | Keizer | Cesarz | Kaiser | Kejsare | ไกเซอร์ | 皇帝