Grey literature is defined as: "...information produced on all levels of government, academia, business and industry in electronic and print formats not controlled by commercial publishing (i.e. where publishing is not the primary activity of the producing body) --(ICGL Luxembourg definition, 1997) (Expanded in New York, 2004)
"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, i.e. where publishing is not the primary activity of the producing body." (Schopfel, 2010)
Paez, A. Grey literature: An important resource in systematic reviews. J Evid Based Med. 2017 Dec 21. doi: 10.1111/jebm.12265. Epub ahead of print. PMID: 29266844.