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]