Grey literature can be described in many ways, but the most widely accepted definition was adopted in 1997 and expanded in 2004 at the Grey Literature Conference in Luxembourg and New York City, respectively. This definition states that grey literature is "information produced on all levels of government, academia, business and industry in electronic and print formats not controlled by commercial publishing, i.e. where publishing is the not the primary activity of the producing body" (Mering, 2018). Due to its often informal nature, grey literature is seldom subjected to a peer review process, and as such can vary widely in quality.
Examples of grey literature include, but are not limited to:
Mering, M. (2018). Defining and Understanding Grey Literature. Serials Review, 44(3), 238–240. https://doi.org/10.1080/00987913.2018.1525239