Gray literature is a concept that refers to a wide variety of unpublished or informally published information from different groups such as professional associations, research institutes, government agencies, and non-profit organizations. It is usually difficult to track down this type of information because it is not published commercially (e.g. in a research journal), nor is it widely accessible.
For a more in-depth discussion of the definition of gray literature, please see "Toward a Prague Definition of Grey Literature" by Joachim Schoepfel, specifically page 18.
Gray literature comes in many different forms. Some examples include:
Although it may be difficult to find, gray literature can be an important part of your research because it:
Like all information, gray literature has some potential weaknesses to be aware of: