I like this definition: "There are two types of ethically invalid land titles: "feudalism," in which there is continuing aggression by titleholders of land against peasants engaged in transforming the soil; and land-engrossing, where arbitrary claims to virgin land are used to keep first-transformers out of that land. We may call both of these aggressions "land monopoly"—not in the sense that some one person or group owns all the land in society, but in the sense that arbitrary privileges to land ownership are asserted in both cases, clashing with the libertarian rule of non-ownership of land except by actual transformers, their heirs, and their assigns." — Murray N. Rothbard, The Ethics of Liberty