The Collaborative International Dictionary of English v.0.48:

agency \a"gen*cy\ ([=a]"jen*s[y^]), n.; pl. Agencies
   ([=a]"jen*s[i^]z). [agentia, fr. L. agens, agentis: cf. F.
   agence. See Agent.]
   1. The faculty of acting or of exerting power; the state of
      being in action; action; instrumentality.
      [1913 Webster]

            The superintendence and agency of Providence in the
            natural world.                        --Woodward.
      [1913 Webster]

   2. The office of an agent, or factor; the relation between a
      principal and his agent; business of one intrusted with
      the concerns of another.
      [1913 Webster]

   3. The place of business of an agent.
      [1913 Webster]

   4. An organization or business that provides some service,
      primarily for other organizations; as, an advertising
      agency.
      [PJC]

   5. An administrative division of a government; as, a state
      welfare agency; a motor vehicle agency; the Environmental
      Protection Agency (EPA); the Federal Emergency Management
      Agency (FEMA).
      [PJC]

   Syn: Action; operation; efficiency; management.
        [1913 Webster]

WordNet (r) 3.0 (2006):

agency
    n 1: an administrative unit of government; "the Central
         Intelligence Agency"; "the Census Bureau"; "Office of
         Management and Budget"; "Tennessee Valley Authority" [syn:
         agency, federal agency, government agency, bureau,
         office, authority]
    2: a business that serves other businesses
    3: the state of being in action or exerting power; "the agency
       of providence"; "she has free agency"
    4: the state of serving as an official and authorized delegate
       or agent [syn: representation, delegacy, agency]
    5: how a result is obtained or an end is achieved; "a means of
       control"; "an example is the best agency of instruction";
       "the true way to success" [syn: means, agency, way]