A reversion in property law is a future interest that is retained by the grantor after the conveyance of an estate of a lesser quantum than he has (such as the owner of a fee simple granting a life estate or a leasehold estate). Once the lesser estate comes to an end (the lease expires or the life estate tenant dies), the property automatically reverts (hence reversion) back to the grantor.
Unlike private-law reversion, territorial reversion concerns sovereign rights over states or principalities and was common in early modern and nineteenth-century Europe. It was is a principle of public and dynastic law by which a territory returns to a previous sovereign or ruling house upon the extinction of a ruling line or the occurrence of a condition provided for in treaties, dynastic statutes, or acts of investiture.