"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." (from the Fourth International Conference on Grey Literature: New Frontiers in Grey Literature.GreyNet, Grey Literature Network Service. Washington D.C. USA, 4-5 October 1999.)
"The term ‘grey literature’ is often used to refer to reports published outside of traditional commercial publishing. In particular, review authors consider searching sources such as dissertations and conference abstracts."Cochrane Handbook for Systematic Reviews of Interventions, Version 6.4, 2023,https://training.cochrane.org/handbook/current/chapter-04#section-4-3-5
A summary of the various document types which constitute gray literature is available from GreyNet, an organization dedicated to tracking gray literature.