A municipal bond is a bond issued by a sovereign (including Indian tribal governments), local government, territory of the United States, or their agencies. The term municipal bond is commonly used in the United States, which has the largest market of such trade-able securities in the world estimated at $3.7 Trillion in 2011. Potential issuers of municipal bonds include states, cities, counties, redevelopment agencies, special-purpose districts, school districts, public utility districts, publicly owned airports and seaports, and any other governmental entity (or group of governments) at or below the state level having more than a de minimis amount of one of the three sovereign powers: the power of taxation, the power of eminent domain or the police power. Municipal bonds may be general obligations of the issuer or secured by specified revenues.