Grey literature is generally defined as academic literature that is not formally published. It is an expanding field in library and information science that deals with the supply and demand side of publications not controlled by commercial publishing. (1) There may be much more grey literature than published literature, given the fact that many fewer manuscripts of articles and books are accepted for publication than are submitted to journal or book publishers.
Research in this field of information has been systematically documented and archived via the International Conference Series on Grey Literature (1993, Vol.1)...(2014, Vol.16) and The Grey Journal (2005, Vol.1)...(2014, Vol.10) a flagship journal for the grey literature community. On 16 May 2014, the Pisa Declaration on Policy Development for Grey Literature Resource was ratified and published. (1)
References and further information:
1. Wikipedia page on grey literature: http://en.wikipedia.org/wiki/Grey_literature