The Collaborative International Dictionary of English v.0.48:

Tyranny \Tyr"an*ny\ (t[i^]r"an*n[y^]), n. [OE. tirannye, OF.
   tirannie, F. tyrannie; cf. It. tirannia; Gr. tyranni`a,
   tyranni`s, L. tyrannis. See Tyrant.]
   1. The government or authority of a tyrant; a country
      governed by an absolute ruler; hence, arbitrary or
      despotic exercise of power; exercise of power over
      subjects and others with a rigor not authorized by law or
      justice, or not requisite for the purposes of government.
      [1913 Webster]

            "Sir," would he [Seneca] say, "an emperor mote need
            Be virtuous and hate tyranny."        --Chaucer.
      [1913 Webster]

   2. Cruel government or discipline; as, the tyranny of a
      schoolmaster.
      [1913 Webster]

   3. Severity; rigor; inclemency.
      [1913 Webster]

            The tyranny of the open night's too rough
            For nature to endure.                 --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

tyranny
    n 1: a form of government in which the ruler is an absolute
         dictator (not restricted by a constitution or laws or
         opposition etc.) [syn: dictatorship, absolutism,
         authoritarianism, Caesarism, despotism, monocracy,
         one-man rule, shogunate, Stalinism,
         totalitarianism, tyranny]
    2: dominance through threat of punishment and violence [syn:
       absolutism, tyranny, despotism]

Moby Thesaurus II by Grady Ward, 1.0:

125 Moby Thesaurus words for "tyranny":
   Caesarism, Nazism, Stalinism, absolute monarchy, absolutism,
   arbitrariness, argumentum baculinum, aristocracy, autarchy,
   authoritarianism, autocracy, autonomy, benevolent despotism,
   big battalions, big stick, bond service, bondage, brute force,
   captivity, coalition government, colonialism, commonwealth,
   constitutional government, constitutional monarchy, control,
   czarism, debt slavery, democracy, deprivation of freedom,
   despotism, dictatorship, disenfranchisement, disfranchisement,
   domination, domineering, dominion rule, duarchy, duumvirate,
   dyarchy, enslavement, enthrallment, fascism, federal government,
   federation, feudal system, feudalism, feudality, force,
   garrison state, gerontocracy, heavy hand, helotism, helotry,
   heteronomy, hierarchy, hierocracy, high hand, home rule,
   indentureship, iron boot, iron hand, iron heel, kaiserism,
   limited monarchy, main force, martial law, meritocracy, militarism,
   military government, mob rule, mobocracy, monarchy, monocracy,
   naked force, neocolonialism, ochlocracy, oligarchy, one-man rule,
   one-party rule, oppression, pantisocracy, paternalism,
   patriarchate, patriarchy, peonage, physical force, police state,
   pure democracy, regency, reign of terror, representative democracy,
   representative government, republic, restraint, rule of might,
   self-determination, self-government, serfdom, serfhood, servility,
   servitude, slavery, social democracy, steamroller, stratocracy,
   subjection, subjugation, suppression, technocracy, terrorism,
   thearchy, theocracy, thought control, thrall, thralldom,
   totalitarian government, totalitarian regime, totalitarianism,
   totality, triarchy, triumvirate, ultima ratio, vassalage,
   villenage, welfare state



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

TYRANNY, government. The violation of those laws which regulate the division
and the exercises of the sovereign power of the state. It is a violation of
its constitution.