What is Grey Literature?
Grey Literature (or gray literature) is a term used ... to refer to a body of materials that cannot be found easily through conventional channels such as publishers, "but which is frequently original and usually recent” (Debachere, M. C. (1995). "Problems in obtaining grey literature". IFL4 Journal 21 (2): 94–98. doi:10.1177/034003529502100205)
. ... Examples of grey literature include technical reports from government agencies or scientific research groups, working papers from research groups or committees, white papers, or preprints. The term grey literature is often, but not exclusively, used for scientific research.