Irredentism is the doctrine of political or popular movements that claim and seek to occupy (usually on behalf of their members' nation) territory considered "lost" (or "unredeemed") to the nation, based on history or legend. The scope is occasionally subject to disputes about underlying claims of expansionism, owing to lack of clarity on the historical bounds of putative nations or peoples.
This term also often refers to revanchism, though the difference between the two is, according to Merriam-Webster, that the irredentism is the reunion of politically or ethnically displaced territory, along with a population having the same national identity. On the other hand, "revanchism" evolved from the French word "revanche" which means revenge. In the political realm, revanchism is such a theory that intends to seek revenge for a lost territory.
The word (from Italian irredento for "unredeemed") was coined in Italy from the phrase Italia irredenta ("unredeemed Italy"). This originally referred to rule by Austria-Hungary over territories mostly or partly inhabited by ethnic Italians, such as Trentino, Trieste, Gorizia, Istria, Fiume and Dalmatia during the 19th and early 20th centuries. An area liable to be targeted by a claim is sometimes called an "irredenta".
A common way to express a claim to adjacent territories on the grounds of historical or ethnic association is by using the adjective "Greater" as a prefix to the country name e.g. Greater Germanic Reich, Greater Hungary, Greater Serbia, Greater Albania etc. This conveys the concept and image of national territory at its maximum conceivable or claimed extent, with the country "proper" at its core. The use of "Greater" does not always equate, however, to irredentism; an example of such a situation is Greater Poland, the region containing the historical nucleus of the Polish statehood, which bears the adjective “Greater” in order to distinguish it from the region of Lesser Poland, and is unrelated to any concept of Polish territorial claims.