"Grey literature" is the intellectual output of universities, governmental and non-governmental agencies, foundations, organizations, businesses, institutes, think tanks, and other bodies that is never published. It can take the form of reports, preprints, white papers, working papers, presentations, proceedings, internal documents, abstracts, policies and procedures. These kinds of materials often do not appear in searching the usual scholarly communication venues and may be difficult to locate.
Image courtesy of Carol Shannon's University of Michigan LibGuide on Grey Literature