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