King James Dictionary 
1. Supreme power in governing supreme dominion sovereignty imperial power. No nation can rightfully claim the empire of the ocean. 2. The territory, region or countries under the jurisdiction and dominion of an emperor. An empire is usually a territory of greater extent than a kingdom, which may be and often is a territory of small extent. Thus we say, the Russian empire the Austrian empire the sovereigns of which are denominated emperors. The British dominions are called an empire, and since the union of Ireland, the parliament is denominated the imperial parliament, but the sovereign is called king. By custom in Europe, the empire means the German empire and in juridical acts, it is called the holy Roman empire. Hence, we say, the diet of the empire the circles of the empire &c. But the German empire no longer exists the states of Germany now form a confederacy. 3. Supreme control governing influence rule sway as the empire of reason, or of truth. 4. Any region, land or water, over which dominion is extended as the empire of the sea.
Webster's Dictionary 
(1): ( n.) The dominion of an emperor; the territory or countries under the jurisdiction and dominion of an emperor (rarely of a king), usually of greater extent than a kingdom, always comprising a variety in the nationality of, or the forms of administration in, constituent and subordinate portions; as, the Austrian empire.
(2): ( n.) Supreme power; sovereignty; sway; dominion.
(3): ( n.) Any dominion; supreme control; governing influence; rule; sway; as, the empire of mind or of reason.