"Printed works such as reports, preprints, internal documents, Ph.D. dissertations, master’s theses, and conference proceedings, not readily available through regular market channels because they were never commercially published or listed or were poorly distributed." Reitz, J. M. (2004). Dictionary for Library and Information Science. Libraries Unlimited.
"The gray literature includes unpublished internal company research reports, conference preceedings and abstracts, preliminary works, studies done only for submission to national drug approval agencies, preliminary studies conducted in advance of a full study ..., and studies that have not yet been published or submitted for publication." McKibbon, A., & Nancy Wilczynski. (2009). PDQ Evidence-Based Principles & Practice, 2nd edition PMPH USA.
The Fourth International Conference on Grey Literature (GL '99) in Washington, DC, in October 1999 defined grey literature as follows: "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." Home: Grey Literature Database. (n.d.). Retrieved March 10, 2020, from http://www.greylit.org/
For more info on Grey Literature go to the Grey Literature Report