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.
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)