claiming a right to territories belonging to another state on the grounds of common ethnicity and/or prior historical possession, actual or alleged.

the demands of ethnonationalist groups to take political control of territory historically or ethnically related to them by separating from their parent state or taking territory from other states (127)

Irredentism is an international relations term that involves advocating annexation of territories administered by another state on the grounds of common ethnicity and/or prior historical possession, actual or alleged. It is a feature of identity politics and cultural and political geography. Since most borders have been moved and redrawn at one point, a great many countries could theoretically present irredentist claims to their neighbours.