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.
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   2. A ruling body composed of the best citizens. [Obs.]
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            In the Senate
            Right not our quest in this, I will protest them
            To all the world, no aristocracy.     --B. Jonson.
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   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.
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            The aristocracy of Venice hath admitted so many
            abuses, trough the degeneracy of the nobles, that
            the period of its duration seems approach. --Swift.
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   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.
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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]

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is obsolete; so is that kind of government.)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.