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]

Moby Thesaurus II by Grady Ward, 1.0:

163 Moby Thesaurus words for "dictatorship":
   Caesarism, Stalinism, absolute monarchy, absolutism, aedileship,
   archbishopric, archiepiscopacy, archiepiscopate, aristocracy,
   autarchy, authoritarianism, autocracy, autonomy,
   benevolent despotism, bishopric, captainship, chairmanship,
   chancellery, chancellorate, chancellorship, chiefery, chiefry,
   chieftaincy, chieftainry, chieftainship, coalition government,
   colonialism, commonwealth, constitutional government,
   constitutional monarchy, consulate, consulship, convenership,
   custody, czarism, deanery, democracy, despotism, dictature,
   directorate, directorship, dominion rule, duarchy, duumvirate,
   dyarchy, emirate, episcopacy, federal government, federation,
   feudal system, foremanship, garrison state, generalship,
   gerontocracy, governorship, guardianship, headship, hegemony,
   heteronomy, hierarchy, hierocracy, home rule, intendancy,
   kaiserism, leadership, limited monarchy, lordship, magistracy,
   magistrateship, magistrature, managership, martial law, masterdom,
   mastership, mastery, mayoralty, mayorship, meritocracy,
   metropolitanate, metropolitanship, militarism, military government,
   mob rule, mobocracy, monarchy, neocolonialism, nobility,
   ochlocracy, oligarchy, one-man rule, one-party rule, overseership,
   pantisocracy, papacy, pashadom, pashalic, paternalism,
   patriarchate, patriarchy, police state, pontificality, pontificate,
   popedom, popehood, popeship, prefectship, prefecture, premiership,
   presidency, presidentship, prime-ministership, prime-ministry,
   princedom, princeship, principality, proconsulate, proconsulship,
   proctorship, protectorate, protectorship, provostry, provostship,
   pure democracy, rectorate, rectorship, regency, regentship,
   representative democracy, representative government, republic,
   ruling class, seigniory, self-determination, self-government,
   seneschalship, seneschalsy, sheikhdom, sheriffalty, sheriffcy,
   sheriffdom, shrievalty, social democracy, sovereignty, stewardship,
   stratocracy, superintendence, superintendency, supervisorship,
   suzerainship, suzerainty, technocracy, thearchy, theocracy,
   totalitarian government, totalitarian regime, totalitarianism,
   triarchy, tribunate, triumvirate, tyranny, vizierate, viziership,
   welfare state