The Collaborative International Dictionary of English v.0.48:

Monarchy \Mon"arch*y\, n.; pl. Monarchies. [F. monarchie, L.
   monarchia, Gr. ?. See Monarch.]
   1. A state or government in which the supreme power is lodged
      in the hands of a monarch.
      [1913 Webster]

   2. A system of government in which the chief ruler is a
      monarch.
      [1913 Webster]

            In those days he had affected zeal for monarchy.
                                                  --Macaulay.
      [1913 Webster]

   3. The territory ruled over by a monarch; a kingdom.
      [1913 Webster]

            What scourage for perjury
            Can this dark monarchy afford false Clarence.
                                                  --Shak.
      [1913 Webster]

   Fifth monarchy, a universal monarchy, supposed to be the
      subject of prophecy in Daniel ii.; the four preceding
      monarchies being Assyrian, Persian, Grecian, and Roman.
      See Fifth Monarchy men, under Fifth.
      [1913 Webster]

WordNet (r) 3.0 (2006):

monarchy
    n 1: an autocracy governed by a monarch who usually inherits the
         authority

Moby Thesaurus II by Grady Ward, 1.0:

81 Moby Thesaurus words for "monarchy":
   Caesarism, Stalinism, absolute monarchy, absolutism, aristocracy,
   autarchy, authoritarianism, autocracy, autonomy,
   benevolent despotism, coalition government, colonialism,
   commonwealth, constitutional government, constitutional monarchy,
   country, czarism, democracy, despotism, dictatorship, domain,
   dominion, dominion rule, duarchy, duumvirate, dyarchy, empire,
   federal government, federation, feudal system, garrison state,
   gerontocracy, heteronomy, hierarchy, hierocracy, home rule,
   kaiserism, kingdom, limited monarchy, martial law, meritocracy,
   militarism, military government, mob rule, mobocracy, monarchism,
   monocracy, nation, neocolonialism, ochlocracy, oligarchy,
   one-man rule, one-party rule, pantisocracy, paternalism,
   patriarchate, patriarchy, police state, principality,
   pure democracy, regency, representative democracy,
   representative government, republic, royalism, self-determination,
   self-government, social democracy, sovereignty, state, stratocracy,
   technocracy, thearchy, theocracy, totalitarian government,
   totalitarian regime, totalitarianism, triarchy, triumvirate,
   tyranny, welfare state



Bouvier's Law Dictionary, Revised 6th Ed (1856):

MONARCHY, government. That form of government in which the sovereign power
is entrusted to the hands of a single magistrate. Toull. tit. prel. n. 30.
The country governed by a monarch is also called a monarchy.