“Grey literature is the unpublished, non-commercial, hard-to-find information that organizations such as professional associations, research institutes, think tanks, and government departments produce.”
(https://guides.lib.umich.edu/greyliterature)
Luxembourg definition
“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” (Farace, 1997)