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The Old French word reaume, modern French royaume, was the word first adopted in English; the fixed modern spelling does not appear until the beginning of the 17th century. The word supposedly derives from medieval Latin regalimen, from regalis, of or belonging to a rex, (king).
"Realm" is particularly used for those states whose name includes the word kingdom (for example, the United Kingdom), to avoid clumsy repetition of the word in a sentence (for example, "The Queen's realm, the United Kingdom..."). It is also useful to describe those countries whose monarchs are called something other than "king" or "queen"; for example, the Grand Duchy of Luxembourg is a realm but not a kingdom since its monarch holds the title Grand Duke rather than King.
"Realm" is also frequently used to refer to territories that are "under" a monarch, yet are not a physical part of his or her "kingdom" (e.g. the Realm of Sweden, or to Holstein, which until the Second War of Schleswig was an important part of the Danish King's realm stretching to the border of Hamburg, although not a part of the Danish Kingdom). Similarly, the Cook Islands, Niue, and Tokelau are considered parts of the Realm of New Zealand, although they are not part of New Zealand proper. Likewise, the Faroe Islands and Greenland remain parts of the Danish Realm.
Realm may commonly also be used to describe the Commonwealth realms which all are kingdoms in their own right and share a common monarch, though they are fully independent of each other.
See also 
- Encyclopædia Britannica Eleventh Edition article "Realm"