Grey (Gray) Literature "refers to informally published written material (such as reports) that may be difficult to trace via conventional channels such as published journals and monographs because it is not published commercially or is not widely accessible" (Wikipedia, n.d.)
This can include information such as government reports, conference proceedings, graduate dissertations, unpublished clinical trials, and much more.
Why Grey Literature?
It may provide data not found within commercially published literature, providing an important forum for disseminating studies with null or negative results that might not otherwise be disseminated. Grey literature may thusly reduce publication bias, increase reviews' comprehensiveness and timeliness and foster a balanced picture of available evidence. Grey literature's diverse formats and audiences can present a significant challenge in a systematic search for evidence (Paez, 2017).
Finding & Verifying Grey Literature
Finding grey literature can be difficult and require exhaustive searching. Since grey literature is not included in bibliographic databases, it requires different search techniques in order to find information relevant to your topic of interest. The video below offers a quick introduction, but a librarian is always a great place to start!