Grey Literature is essentially any document that hasn't gone through peer review for a publication. It can also include conference proceedings or doctoral thesis.
There are several formal definitions. The Twelfth International Conference on Grey Literature in Prague in 2010 arrived at the following definition:
"Grey literature stands for manifold document types produced on all levels of government, academics, business and industry in print and electronic formats that are protected by intellectual property rights, of sufficient quality to be collected and preserved by libraries and institutional repositories, but not controlled by commercial publishers; i.e. where publishing is not the primary activity of the producing body."
Grey literature can be more current than scholarly articles because it doesn’t necessarily go through a rigorous review process so information can be produced and disseminated quickly. It’s important to know how to evaluate grey literature for your academic assignments to make sure what your citing is reliable.
Grey literature produces information on non-mainstream or niche research not typically printed by commercial publishers.
Grey literature can be found for free on the Web, but they can be a challenge to find!
What are the examples of Grey Literature?
conference abstracts,
pre-print & post-print,
presentations/posters,
proceedings,
regulatory data,
unpublished trial data,
government publications,
reports (such as white papers, working papers, internal documentation),
patents
policies & procedures.