There are many definitions of grey literature, but it is usually understood to mean literature that is not formally published in sources such as books or journal articles. It may be described as ephemeral, invisible, informal, underground, etc., that is, literature that may be unevaluated, not peer-reviewed.
Grey Literature exists in many formats: reports-including preprints; preliminary progress and advanced reports; institutional, internal, technical, and statistical reports; research memoranda; state-of-the-art reports; market research reports; reports of commissions and study groups; as well as
* conference proceedings
* technical specifications and standards
* translations (not distributed commercially)
* technical and commercial documentation
* official documents (issued in limited numbers)