Grey literature is an umbrella term for types of information that are produced outside of the commercial publication system and distributed irregularly or informally. Many types of information have been classified as grey literature.  These types of information share characteristics in common:

• They are frequently funded by governmental agencies, via grant awards, contracts, or in support of legislative or executive actions.
• They report all findings of a project or program, rather than simply reporting favorable results (and bypass the phenomena of "publication bias", the tendency of commercial publishers to publish items that show positive results, in order to enhance the impact of that journal within a discipline.
• They focus on actionable information (whether policy, purchasing, immediate innovations in research, requirements for processes, etc.) 
• They have an institutional author or publisher.
• They were not indexed in traditional discovery systems on a consistent basis.
• They have inconsistent identifiers, across types of grey literature and within a given series.
• They are generally published as monographic serials or standalone items.
  • Monographic serials - Items that are released chronologically, and share a serial title, but have differing item level titles from volume to volume.
• They do not go through a formal peer review process.

Defining Grey Literature

In 2024, the following text can be found on the Library of Congress' website:

"Depending on one's definition of a technical report, the number of reports held by the Library runs into the millions. Of these, over 5 million are housed in the Science Section's Technical Reports Collections, held in print, or on microform or digital media.... Technical Reports housed in the Science Section's Technical Reports Collections are generally not cataloged. Researchers looking for information on them may consult Library of Congress Databases and Electronic Resources to locate online indexes for technical reports and standards. Please note, many of the resources listed are available onsite only. Reports located in the General Collections do have catalog records, which can be found in the Library of Congress Online Catalog." (https://guides.loc.gov/technical-reports/technical-reports)"

This paragraph about the preeminent library collection in the United States clearly shows the historical problems of accessing grey literature. The term "grey literature" was first developed by Charles P. Auger, when reflecting on the discoverability and persistence of  intelligence reports and research reports on atomic research produced during World War II.

A formal definition was developed and approved at the 3rd International Conference on Grey Literature in 1997. Grey literature was defined as ""that which is produced on all levels of government, academics, business and industry in print and electronic formats, but which is not controlled by commercial publishers". In 2004, an additional caveat was added: "i.e., where publishing is not the primary activity of the producing body."

Schöpfel, J.; Farace, D.J. (2010). "Grey Literature". In Bates, M.J.; Maack, M.N. (eds.). Encyclopedia of Library and Information Sciences (3rd ed.). Boca Raton, Fla.: CRC Press. pp. 2029–2039. ISBN 978-0-8493-9712-7.