Grey (or gray) literature is:
Source: GreyNet International
It's called grey literature because it can be fuzzy, hard to define, and (in many cases) hard to locate.
Examples of grey literature include:
Using grey literature will help strengthen your research. It can be more current than the literature found in books/journals and thus offer you a fuller picture of the information surrounding your topic. When you search for grey lit, expect to run many searches and click on many links.
Full text access to technical literature in electrical engineering, computer science, and electronics. Contains documents from IEEE journals, transactions, magazines, letters, conference proceedings, standards, and IEE (Institution of Electrical Engineers) publications. Not compatible with IE browsers.