I've seen this
variously translated as "Land of
Islam", "
Realm of Peace", and "
Domain of Justice". Well, "land", "
realm", and "
domain" are all pretty close to each other, and I'd be surprised if the word "
Islam" could be translated literally and exactly into
English, so there you go.
Anyhow, the term is very precisely defined in
Islamic legal rulings; one source says this:
"Dar al-Islam is a territory ruled by an Islamic Amir al-Mu'minin (leader of the Believers, a Caliph that is accepted as such by the consensus of Islamic scholars) under the Jurisdiction of the Shari'ah."1
However, I've most often see it used more loosely, as a rough
analogue of "
Christendom", referring to the sum of all
Islamic-controlled territory at the height of
Islamic world power (around something like roughly more or less the ninth through fourteenth centuries, give or take). Perhaps more so than in "
Christendom", the Land of
Islam was seen as an integral unit, with less cultural emphasis placed on
national boundaries than we see in the modern world. In some ways, this sense of things hasn't entirely died out; I've read that there was great sorrow in the
Islamic world over the
Iran-Iraq War, because they were both
Islamic nations.
If I've gotten something wrong here,
for God's sake (pick your favorite
God) let me know.
1 http://www.rb.org.il/islam/commentary/islamcom16.htm