Grey literature consists of documents produced by government, academic, business or organizations "where publishing is not the primary activity of the producing body." (Greynet)
Examples include annual reports, conference proceedings, technical reports, theses, white papers, and even informal communication such as blogs, emails, or social media posts.
For systematic and scoping reviews, a grey literature search may be mandatory or strongly recommended. For instance, Cochrane's MECIR standards require authors to: "Search relevant grey literature sources such as reports, dissertations, theses, databases and databases of conference abstracts. Searches for studies should be as extensive as possible in order to reduce the risk of publication bias and to identify as much relevant evidence as possible."
For other research projects, the grey literature may be: