A thalassocracy or thalattocracy, sometimes also maritime empire, is a state with primarily maritime realms, an empire at sea, or a seaborne empire. Traditional thalassocracies seldom dominate interiors, even in their home territories. Examples of this were the Phoenician states of Tyre, Sidon and Carthage; the Italian maritime republics of Venice and Genoa of the Mediterranean; the Chola dynasty of Tamil Nadu in India; the Omani Empire of Arabia; and the Austronesian empires of Srivijaya and Majapahit in Maritime Southeast Asia. Thalassocracies can thus be distinguished from traditional empires, where a state's territories, though possibly linked principally or solely by the sea lanes, generally extend into mainland interiors in a tellurocracy.

An empire is a political unit made up of several territories, military outposts, and peoples, "usually created by conquest, and divided between a dominant center and subordinate peripheries". The center of the empire exercises political control over the peripheries. Within an empire, different populations have different sets of rights and are governed differently. Narrowly defined, an empire is a sovereign state whose head of state is an emperor or empress; but not all states with aggregate territory under the rule of supreme authorities are called empires or are ruled by an emperor; nor have all self-described empires been accepted as such by contemporaries and historians.