Definitions of circulation:

• noun:   the spread or transmission of something (as news or money) to a wider group or area
• noun:   the dissemination of copies of periodicals (as newspapers or magazines)
• noun:   free movement or passage through a series of vessels (as of water through pipes or sap through a plant)
• noun:   movement through a circuit; especially the movement of blood through the heart and blood vessels
• noun:   number of copies of a newspaper or magazine that are sold

  Example: "By increasing its circulation the newspaper hoped to increase its advertising"

• noun:   (library science) the count of books that are loaned by a library over a specified period