From The Art and Popular Culture Encyclopedia
A newspaper's circulation is the number of copies it distributes on an average day. Newspaper circulation rates are currently experiencing a downward trend. Circulation is one of the principal factors used to set advertising rates. Circulation is not always the same as copies sold, often called paid circulation, since some newspapers are distributed without cost to the reader. Readership figures are usually higher than circulation figures because of the assumption that a typical copy of the newspaper is read by more than one person.
In many countries, circulations are audited by independent bodies such as the Audit Bureau of Circulations to assure advertisers that a given newspaper does indeed reach the number of people claimed by the publisher.