"Grey literature" (or "gray literature") is a term used to describe a variety of materials that fall outside the traditional publishing and distribution channels.
Grey literature can be produced by government agencies, non-profit organizations, academic institutions and departments, and private companies.
These materials do not go through the same rigorous peer-review process that most scholarly journal articles do, but they can still provide valuable information to supplement peer-reviewed resources. Be mindful to evaluate these sources carefully and only use them when appropriate.
Examples of grey literature include: