"Grey literature stands for manifold document types produced on all levels of government, academics, business and industry in print and electronic formats that are protected by intellectual property rights, of sufficient quality to be collected and preserved by libraries and institutional repositories, but not controlled by commercial publishers; i.e. where publishing is not the primary activity of the producing body." (12th International Conference on Grey Literature, Prague, 2010)
Examples of grey literature include reports, white papers, conference proceedings and theses/dissertations.
Please note that in the United States of America, GRAY is how it is spelled, while in Commonwealth countries, GREY is how it is spelled.