Grey literature "refers to print or electronic literature that is produced by government, academia, business and industry, and is not controlled by commercial publishers ... i.e. where publishing is not the primary activity of the producing body." (ICGL Luxembourg definition, 1997. Expanded in New York, 2004.)
Grey literature does not form a part of traditional publishing, and is not widely disseminated. It can include a huge range of resources form theses and dissertations, through conference proceedings, government documents, and research reports to digital repositories and registers of clinical trials.
For more information, see Charles Sturt Library's guide on Grey Literature.