The Collaborative International Dictionary of English v.0.48:

Dictatorship \Dic*ta"tor*ship\, n.
   The office, or the term of office, of a dictator; hence,
   absolute power.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dictatorship
    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]