countable noun. A dukedom is the land owned by a duke.
What type of noun is duke?
duke used as a noun:
The male ruler of a duchy (compare duchess). A high title of nobility; the male holder of a dukedom. A grand duke.
What is the difference between a duchy and a dukedom?
A duchy is the territory or geopolitical entity ruled by a duke, whereas his title or area is often called a dukedom. The Grand Duchy of Luxembourg is a fully independent state and its head, the Grand Duke, is a sovereign monarch reigning over his Luxembourgish subjects.