What is "kaiserthum"

Kaiserthum or modern German, Kaisertum, is a German word for Empire in its meaning as a state ruled over by an Emperor, used in the 18th and 19th centuries. It is most known as a description of the Empire of Austria after its creation in 1804. A later used German term for it is "Kaiserreich", a term that was first used officially in German in 1915, to describe the Empire of Austria.