The Collaborative International Dictionary of English v.0.48:

authorities \authorities\ n.
   1. the organization that is the governing authority of a
      political unit.

   Syn: government, regime.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Authority \Au*thor"i*ty\, n.; pl. Authorities. [OE. autorite,
   auctorite, F. autorit['e], fr. L. auctoritas, fr. auctor. See
   Author, n.]
   1. Legal or rightful power; a right to command or to act;
      power exercised buy a person in virtue of his office or
      trust; dominion; jurisdiction; authorization; as, the
      authority of a prince over subjects, and of parents over
      children; the authority of a court.
      [1913 Webster]

            Thus can the demigod, Authority,
            Make us pay down for our offense.     --Shak.
      [1913 Webster]

            By what authority doest thou these things ? --Matt.
                                                  xxi. 23.
      [1913 Webster]

   2. Government; the persons or the body exercising power or
      command; as, the local authorities of the States; the
      military authorities. [Chiefly in the plural.]
      [1913 Webster]

   3. The power derived from opinion, respect, or esteem;
      influence of character, office, or station, or mental or
      moral superiority, and the like; claim to be believed or
      obeyed; as, an historian of no authority; a magistrate of
      great authority.
      [1913 Webster]

   4. That which, or one who, is claimed or appealed to in
      support of opinions, actions, measures, etc. Hence:
      (a) Testimony; witness. "And on that high authority had
          believed." --Milton.
      (b) A precedent; a decision of a court, an official
          declaration, or an opinion, saying, or statement
          worthy to be taken as a precedent.
      (c) A book containing such a statement or opinion, or the
          author of the book.
      (d) Justification; warrant.
          [1913 Webster]

                Wilt thou be glass wherein it shall discern
                Authority for sin, warrant for blame. --Shak.
          [1913 Webster]

WordNet (r) 3.0 (2006):

authorities
    n 1: the organization that is the governing authority of a
         political unit; "the government reduced taxes"; "the matter
         was referred to higher authorities" [syn: government,
         authorities, regime]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "authorities":
   John Bull, Uncle Sam, Washington, Whitehall, bureaucracy,
   directorate, hierarchy, higher echelons, higher-ups, management,
   ministry, officialdom, prelacy, ruling class, ruling classes,
   the Crown, the Establishment, the administration, the authorities,
   the government, the ingroup, the interests, the people upstairs,
   the power elite, the power structure, the top, them, they,
   top brass



Bouvier's Law Dictionary, Revised 6th Ed (1856):

AUTHORITIES, practice. By this word is understood the citations which are
made of laws, acts of the legislature, and decided cases, and opinions of
elementary writers. In its more confined sense, this word means, cases
decided upon solemn argument which are said to 'be authorities for similar
judgments iii like cases. 1 Lilly's Reg. 219. These latter are sometimes
called precedents. (q.v.) Merlin, Repertoire, mot Autorites.
     2. It has been remarked, that when we find an opinion in a text writer
upon any particular point, we must consider it not merely as the opinion of
the author, but as the supposed result of the authorities to which he
refers; 3 Bos. & Pull. 361; but this is not always the case, and frequently
the opinion is advanced with the reasons which support it, and it must stand
or fall as these are or are not well founded. A distinction has been made
between writers who have, and those who have not holden a judicial station;
the former are considered authority, and the latter are not so considered
unless their works have been judicially approved as such. Ram. on Judgments,
93. But this distinction appears not to be well founded; some writers who
have occupied a judicial station do not possess the talents or the learning
of others who have not been so elevated, and the works or writings of the
latter are much more deserving the character of an authority than those of
the former. See 3 T. R. 4, 241.