The Collaborative International Dictionary of English v.0.48:

PAC \PAC\ (p[a^]k), n. [Acronynm from Political Action
   Committee.] (Politics)
   A political action committee, a committee formed by an
   organization or special-interest group to raise money to
   support candidates for office or to influence legislation. A
   PAC provides a legal means for corporations in the U. S. to
   support political candidates even when direct contributions
   from corporations to candidates is forbidden by law. In
   theory, the corporation may not itself contribute to a PAC,
   but may pay the expenses of raising money from individuals.
   PACs may also be formed by organizations other than
   commercial corporations, such as trade associations.
   [Acronym, U. S.]

   Note: PACs became popular in the 1970's after campaign
         finance reform laws put limits on the amount of money
         which an individual can contribute to each candidate
         for public office. In addition to simply supporting
         candidates with specific viewpoints, the unstated
         purpose of PACs is to make politicians aware of their
         viewpoints, by aggregating sums of money into
         significant single donations. This latter effect has
         aroused criticism of PACs from reformers who feel that
         large donations bias the political process.
         [PJC]

WordNet (r) 3.0 (2006):

political action committee
    n 1: committee formed by a special-interest group to raise money
         for their favorite political candidates [syn: political
         action committee, PAC]