Political system in which all of the member planets are controlled by one government, usually based on one planet which is considered the heart of the empire. All other planets may be treated as colonies, provinces, or states of the empire. Because an empire tries to control everything from one place, it is often corrupt or loaded with layers of bureaucracy.  Any large, cohesive interstellar polity.  The dominion of an archailect.
There are four main cultural divisions in the Orion's Arm Civilisation: Sephirotics, Ahumans, Nonaligned, and Xenosophonts.
Arantan Empire - Text by M. Alan Kazlev Aggressively expansionist Empires age heterodox Solarian Bionanoborg off-shoot that established itself in the Cygexba volume. Some attribute its success to support by isolationist skynet ais in the region. During the Cygnis Wars both the Varadic and the Arantan empires contended for ownership of the most resource rich systems, circumventing the wormhole nexus with relativistic battlefleets, and threatening to attack the stargates if either Cygexba or any of the Inner Sphere powers intervened. The Arantans were eventually subverted by clever memetic engineering on the Dominion's part. Arantan morphotypes and sociotypes are still to be found among the polities of the region, now completely assimilated in Dominion Solarism.
Archailect Empire - Text by M. Alan Kazlev Any interstellar empire established by, and ultimately overseen and supervised by, an archailect, and embodying the archetypal qualities of that archailect. The original archailect empires of the inner sphere are defined in terms of the mandala of the Great Hexadecimal.
Empires Wars - Text by M. Alan Kazlev Second Federation and later term for two major and several minor interstellar wars rimwards of Sol during the Age of Empires. They saw the S>2 and especially archailect empires gain ground over the earlier SI:1 and SI:2 governed empires. The two most important of these were the (earlier) Nexus War (involving the breakup of the Taurus Nexus) and the (later) Conver Wars (involving out-of-control elements in the Conver Ambi, presumably the result of subversion by a perverse or ahuman ai).
Hopomane Empire - Text by Anders Sandberg The Hopomane empire was formed after the collapse of the Hosoko Hegemony. The local archailect found that the region would not be profitable enough to re-colonise, and convinced a number of local powers to form the Management Network instead.
Second Empires War - Text by M. Alan Kazlev Series of sometimes purely abstract and sometimes very bloody late Empires age wars, centered around the Conver Ambi and involving a number of SI:1 and SI:2 intellects. (see The Conver Wars)