Realm is an old term still used as an alternative word for kingdom. It is particularly used for those states which use the word Kingdom in the title (for example, the United Kingdom), to avoid clumsy repetition of the word kingdom in a sentence. For example The Queen's realm, the United Kingdom . . .
It is also frequently used to refer to territories "under" a monarch, yet not a physical part of his or her "kingdom"; for example, the various Commonwealth Realms under the British crown, in Realm of Sweden, or to Holstein that until the Second War on 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, though they are not part of New Zealand proper. Likewise, the Faroe Islands and Greenland remain parts of the Danish Realm.
In Java-J2EE realm terms a database containing users, usergroups and their roles (sets of permissions to access server-resources). Optionally a realm manages user-passwords, certificates and authentication logic.
"Realm" is also the collection of epic stories written by John "Gorgeous" Allen; a cult-favorite sci-fi/fantasy author originating from Wilmington, NC. The Realm series chronicles the history of a universal organization devoted to the protection and defense of the innocent, the organization being The Circle--this organization pre-dating the time of earhtly civilization and finding its origins of angelic ancestry. Many of the Realm books orbit around fantastic sagas and Byronic hero figures; most of which champion such motifs as sacrifice and struggling hope.