In October 1999, at the Fourth International Conference on Grey Literature in Washington, DC, grey literature was defined as "that which is produced on all levels of government, academics, business and industry in print and electronic formats, but which is not controlled by commercial publishers."
Grey literature includes many kinds of publications other than traditional books and scholarly journals, including:
Grey literature is usually published and disseminated quickly. Some forms of grey literature are produced by scholars, but rarely go through a peer-review process.
For more information on grey literature, see What is Grey Literature? by the New York Academy of Medicine.