Grey literature can be defined as "multiple document types produced on all levels of government, academics, business, and organization in electronic and print formats not controlled by commercial publishing i.e. where publishing is not the primary activity of the producing body." -Greynet.org
This includes information such as government data, NGO reports, and patents.
For systematic and scoping reviews, a grey literature search may be mandatory or strongly recommended. For instance, Collaboration for Environmental Evidence tells authors that: "Searches for grey literature should normally be included in evidence synthesis for two main reasons: 1) to try to minimize possible publication bias (Hopewell et al. 2007)... and 2) to include studies not intended for the academic domain, such as practitioner reports and consultancy documents which may nevertheless contain relevant information such as details on study methods or results not reported in journal articles often limited by word length."
For other research projects, the grey literature may be:
Contact your subject librarian for tailored advice about finding grey literature for your research topic. The document below guides you through the general process and presents several strategies.
If you plan to publish your work in a particular journal, you might check how other reviews published in that journal have described their grey literature search strategies.
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.