NOUN (6)

1. the dissemination of copies of periodicals (as newspapers or magazines);

2. movement through a circuit; especially the movement of blood through the heart and blood vessels;

3. (library science) the count of books that are loaned by a library over a specified period;

4. number of copies of a newspaper or magazine that are sold;
- Example: "by increasing its circulation the newspaper hoped to increase its advertising"

5. free movement or passage (as of cytoplasm within a cell or sap through a plant);
- Example: "ocean circulation is an important part of global climate"
- Example: "a fan aids air circulation"

6. the spread or transmission of something (as news or money) to a wider group or area;

The Collaborative International Dictionary of English v.0.48:

Circulation \Cir`cu*la"tion\, n. [L. circulatio: cf. F.
   circulation.]
   1. The act of moving in a circle, or in a course which brings
      the moving body to the place where its motion began.
      [1913 Webster]

            This continual circulation of human things. --Swift.
      [1913 Webster]

   2. The act of passing from place to place or person to
      person; free diffusion; transmission.
      [1913 Webster]

            The true doctrines of astronomy appear to have had
            some popular circulation.             --Whewell.
      [1913 Webster]

   3. Currency; circulating coin; notes, bills, etc., current
      for coin.
      [1913 Webster]

   4. The extent to which anything circulates or is circulated;
      the measure of diffusion; as, the circulation of a
      newspaper.
      [1913 Webster]

   5. (Physiol.) The movement of the blood in the blood-vascular
      system, by which it is brought into close relations with
      almost every living elementary constituent. Also, the
      movement of the sap in the vessels and tissues of plants.
      [1913 Webster]

WordNet (r) 3.0 (2006):

circulation
    n 1: the dissemination of copies of periodicals (as newspapers
         or magazines)
    2: movement through a circuit; especially the movement of blood
       through the heart and blood vessels
    3: (library science) the count of books that are loaned by a
       library over a specified period
    4: number of copies of a newspaper or magazine that are sold;
       "by increasing its circulation the newspaper hoped to
       increase its advertising"
    5: free movement or passage (as of cytoplasm within a cell or
       sap through a plant); "ocean circulation is an important part
       of global climate"; "a fan aids air circulation"
    6: the spread or transmission of something (as news or money) to
       a wider group or area

Moby Thesaurus II by Grady Ward, 1.0:

158 Moby Thesaurus words for "circulation":
   Rh factor, Rh-negative, Rh-positive, Rh-type, Rhesus factor,
   advertisement, airing, angular momentum, angular motion,
   angular velocity, announcement, antibody, antigen, arterial blood,
   axial motion, bandying, blood, blood bank, blood cell, blood count,
   blood donor, blood donor center, blood group, blood grouping,
   blood picture, blood platelet, blood pressure, blood serum,
   blood substitute, bloodmobile, bloodstream, book, bowling,
   broadcast, broadcasting, bruiting, bruiting about, centrifugation,
   circling, circuit, circuition, circuitousness, circuitry,
   circularity, circumambience, circumambiency, circumambulation,
   circumflexion, circumgyration, circumlocution, circummigration,
   circumnavigation, circumrotation, circumvolution, clinical dextran,
   course, demonetization, devaluation, deviance, deviancy, deviation,
   deviousness, dextran, diffusion, digression, display,
   dissemination, distribution, erythrocyte, evulgation, excursion,
   excursus, flow, flowing, full circle, globulin, gore, grume,
   gyration, gyre, gyring, hematics, hematologist, hematology,
   hematoscope, hematoscopy, hemocyte, hemoglobin, hemometer, humor,
   ichor, indirection, isoantibody, issuance, issue, issuing,
   leukocyte, lifeblood, meandering, monetization, motion, neutrophil,
   opsonin, orbit, orbiting, passage, periodical, phagocyte, pivoting,
   plasma, plasma substitute, printing, proclamation, promulgation,
   pronouncement, propagation, publication, publishing, red corpuscle,
   reeling, revaluation, revolution, roll, rolling, rotation,
   rotational motion, round, roundaboutness, rounding, serum, spin,
   spinning, spiral, spiraling, spread, spreading, spreading abroad,
   swinging, swirling, swiveling, telecasting, transmission, trolling,
   trundling, turbination, turn, turning, type O, venous blood,
   ventilation, volutation, volution, wheel, wheeling, whir, whirl,
   whirling, white corpuscle