One of the administrative units or provinces of the Achaemenid / Persian empire, e.g. Ionia, Media, Bactria. Each was ruled by a governor or satrap appointed by the king. Sometimes a satrapy became an appanage of the royal princes. A satrapy would consist of a collection of ethnic groups rather than a piece of land with precise boundaries. Inhabitants had to make payments (tribute) to the empire and provide military services. The division of the empire into satrapies was completed by Darius I (reigned 522-486 BC), who established 20 total. The satrapal administration was retained by Alexander III the Great and his successors.