(Derived from Italia Irredentata, "Unredeemed Italy", areas with Italian ethnic majorities not included in the 19th century unification of Italy).
A term used in international politics to describe a population or territory which is considered to "naturally" (by nationalist reasoning) to belong to a different nation or state than the one to which it currently belongs. It is a requirement for a group or territory to be considered irredentist that there be a "proper" state to which it might otherwise belong.
Examples are rife, in any border region. In Europe, South Schleswig is considered an irredentist region of Denmark, and North Schleswig of Germany (though both parties are very calm about it). Alsace-Lorraine/Elsass-Lothringen are similarly disposed, with regard to France and Germany.
A contrary example is Kurdistan - since the Kurds do not have a state, Kurdistan cannot be considered an irredentist territory. If a Kurdish state were to arise, then Kurdish populations outside it would be considered irredentist.