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 territories in Principality of Lyonesse. 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 which all are Duchies, Earldoms, and Baronies in their own right and share a common monarch, though they are fully independent of each other.
Realm of the Principality of Lyonesse
The Principality's Realm is two fold, being made up of possessed Lands then those that are claimed under the Realm and Commonwealth territories. Take for instance, the Principality claims all the lands upon the Atlantic Ocean seafloor once encompassing the Ancient Island Kingdom of Lyonesse (Albion). These lands are vast but, uninhabitable for they lie at the bottom of the sea yet, our ancient honor and claim lie with it so the claim must be made. Within this claim lies the Isles of Scilly, currently a possession of the United Kingdom. Family owned lands are claimed by the Principality, encompassing the northern edge of the Great Angola Bay Swamplands and Game Preserve, federally protected lands of the United States Government yet the claims are made and the Principality was created. Other lands have come into the possession of the Principality with new citizens being added under the Lyonesse Families and Lands Expansion Initiative.