The Collaborative International Dictionary of English v.0.48:

Aristocracy \Ar`is*toc"ra*cy\, n.; pl. Aristocracies. [Gr. ?;
   ? best + ? to be strong, to rule, ? strength; ? is perh. from
   the same root as E. arm, and orig. meant fitting: cf. F.
   aristocratie. See Arm, and Create, which is related to
   Gr. ?.]
   1. Government by the best citizens.
      [1913 Webster]

   2. A ruling body composed of the best citizens. [Obs.]
      [1913 Webster]

            In the Senate
            Right not our quest in this, I will protest them
            To all the world, no aristocracy.     --B. Jonson.
      [1913 Webster]

   3. A form a government, in which the supreme power is vested
      in the principal persons of a state, or in a privileged
      order; an oligarchy.
      [1913 Webster]

            The aristocracy of Venice hath admitted so many
            abuses, trough the degeneracy of the nobles, that
            the period of its duration seems approach. --Swift.
      [1913 Webster]

   4. The nobles or chief persons in a state; a privileged class
      or patrician order; (in a popular use) those who are
      regarded as superior to the rest of the community, as in
      rank, fortune, or intellect.
      [1913 Webster]

WordNet (r) 3.0 (2006):

aristocracy
    n 1: a privileged class holding hereditary titles [syn:
         nobility, aristocracy]
    2: the most powerful members of a society [syn: gentry,
       aristocracy]

Moby Thesaurus II by Grady Ward, 1.0:

196 Moby Thesaurus words for "aristocracy":
   FFVs, absolute monarchy, aedileship, ancestry, ancienne noblesse,
   archbishopric, archiepiscopacy, archiepiscopate,
   aristocraticalness, autarchy, autocracy, autonomy, baronage,
   baronetage, barons, beau monde, birth, bishopric, blood,
   blue blood, bon ton, carriage trade, chairmanship, chancellery,
   chancellorate, chancellorship, chiefery, chiefry, chieftaincy,
   chieftainry, chieftainship, chivalry, coalition government,
   colonialism, commonwealth, constitutional government,
   constitutional monarchy, consulate, consulship, county, cream,
   deanery, democracy, dictatorship, dictature, directorship,
   distinction, dominion rule, duarchy, duumvirate, dyarchy, elect,
   elite, emirate, episcopacy, establishment, federal government,
   federation, feudal system, flower, garrison state, genteelness,
   gentility, gentry, gerontocracy, governorship, haut monde,
   headship, hegemony, heteronomy, hierarchy, hierocracy, high life,
   high society, home rule, honorable descent, jet set, knightage,
   leadership, limited monarchy, lords of creation, lordship,
   magistracy, magistrateship, magistrature, martial law, masterdom,
   mastership, mastery, mayoralty, mayorship, meritocracy,
   metropolitanate, metropolitanship, militarism, military government,
   mob rule, mobocracy, monarchy, neocolonialism, nobility,
   noble birth, nobleness, noblesse, noblesse de robe, ochlocracy,
   old nobility, oligarchy, overlapping, pantisocracy, papacy,
   pashadom, pashalic, patriarchate, patriarchy, patricians,
   patriciate, peerage, police state, pontificality, pontificate,
   popedom, popehood, popeship, power elite, power structure,
   prefectship, prefecture, premiership, presidency, presidentship,
   prime-ministership, prime-ministry, princedom, princeship,
   principality, proconsulate, proconsulship, protectorate,
   protectorship, provostry, provostship, pure democracy, quality,
   rank, rectorate, rectorship, regency, regentship,
   representative democracy, representative government, republic,
   royalty, ruling circles, ruling class, seigniory,
   self-determination, self-government, seneschalship, seneschalsy,
   sheikhdom, sheriffalty, sheriffcy, sheriffdom, shrievalty,
   smart set, social democracy, society, stratocracy, supervisorship,
   suzerainship, suzerainty, technocracy, the Four Hundred, the best,
   the best people, the brass, the classes, thearchy, theocracy,
   top people, totalitarian government, totalitarian regime, triarchy,
   tribunate, triumvirate, tyranny, upper class, upper classes,
   upper crust, upper ten, upper ten thousand, uppercut, vizierate,
   viziership, welfare state



The Devil's Dictionary (1881-1906):

ARISTOCRACY, n.  Government by the best men.  (In this sense the word
is obsolete; so is that kind of government.)  Fellows that wear downy
hats and clean shirts -- guilty of education and suspected of bank
accounts.