What is Grey Literature
The term grey literature is used to describe a wide range of different information that is produced outside of traditional publishing and distribution channels, and which is often not well represented in indexing databases.
A widely accepted definition in the scholarly community for grey literature is
"information produced on all levels of government, academia, business and industry in electronic and print formats not controlled by commercial publishing" ie. where publishing is not the primary activity of the producing body."
From: Third International Conference on Grey Literature in 1997 (ICGL Luxembourg definition, 1997 - Expanded in New York, 2004).
When conducting a systematic review it is important to include literature that has not formally been published in sources as it helps to prevent publication bias. An exhaustive literature search, especially one that leads to a systematic review, needs to include searches in gray literature, to overcome, or at least minimize, “publication bias” or “reporting bias”. Searching grey literature is supported/mandated by the Cochrane Collaboration, the Campbell Collaboration, JBI and the Institute of Medicine (U.S.)