Systematic Reviews: SR Grey Literature

"The information & resources that do not categorically fall into what is available via standard traditional or commercial publishing channels."

Definition by the International Journal on Grey Literature

"Information produced on all levels of government, business & industry in electronic and print format not controlled by commercial publishing, i.e. where publishing is not the primary activity of the producing body."

Definition by  ICGI, Luxemburg definition 1997 - expanded in New York 2004