The Collaborative International Dictionary of English v.0.48:

Commonwealth \Com"mon*wealth`\ (?; 277), n. [Common + wealth
   well-being.]
   1. A state; a body politic consisting of a certain number of
      men, united, by compact or tacit agreement, under one form
      of government and system of laws.
      [1913 Webster]

            The trappings of a monarchy would set up an ordinary
            commonwealth.                         --Milton.
      [1913 Webster]

   Note: This term is applied to governments which are
         considered as free or popular, but rarely, or
         improperly, to an absolute government. The word
         signifies, strictly, the common well-being or
         happiness; and hence, a form of government in which the
         general welfare is regarded rather than the welfare of
         any class.
         [1913 Webster]

   2. The whole body of people in a state; the public.
      [1913 Webster]

   3. (Eng. Hist.) Specifically, the form of government
      established on the death of Charles I., in 1649, which
      existed under Oliver Cromwell and his son Richard, ending
      with the abdication of the latter in 1659.

   Syn: State; realm; republic.
        [1913 Webster]

WordNet (r) 3.0 (2006):

commonwealth
    n 1: the official name of some states in the United States
         (Massachusetts and Pennsylvania and Virginia and Kentucky)
         and associated territories (Puerto Rico)
    2: a politically organized body of people under a single
       government; "the state has elected a new president"; "African
       nations"; "students who had come to the nation's capitol";
       "the country's largest manufacturer"; "an industrialized
       land" [syn: state, nation, country, land,
       commonwealth, res publica, body politic]
    3: a world organization of autonomous states that are united in
       allegiance to a central power but are not subordinate to it
       or to one another
    4: a political system in which the supreme power lies in a body
       of citizens who can elect people to represent them [syn:
       democracy, republic, commonwealth]

The Devil's Dictionary (1881-1906):

COMMONWEALTH, n.  An administrative entity operated by an incalculable
multitude of political parasites, logically active but fortuitously
efficient.

    This commonwealth's capitol's corridors view,
    So thronged with a hungry and indolent crew
    Of clerks, pages, porters and all attaches
    Whom rascals appoint and the populace pays
    That a cat cannot slip through the thicket of shins
    Nor hear its own shriek for the noise of their chins.
    On clerks and on pages, and porters, and all,
    Misfortune attend and disaster befall!
    May life be to them a succession of hurts;
    May fleas by the bushel inhabit their shirts;
    May aches and diseases encamp in their bones,
    Their lungs full of tubercles, bladders of stones;
    May microbes, bacilli, their tissues infest,
    And tapeworms securely their bowels digest;
    May corn-cobs be snared without hope in their hair,
    And frequent impalement their pleasure impair.
    Disturbed be their dreams by the awful discourse
    Of audible sofas sepulchrally hoarse,
    By chairs acrobatic and wavering floors --
    The mattress that kicks and the pillow that snores!
    Sons of cupidity, cradled in sin!
    Your criminal ranks may the death angel thin,
    Avenging the friend whom I couldn't work in.
                                                                  K.Q.