According to the NIH grey (or gray) literature is "...the term for information that falls outside the mainstream of published journal and mongraph literature, not controlled by commercial publishers"
Traditionally, the term grey literature has included:
There are also many other forms of grey literature, including: Newsletters, pamphlets, technical notes, blog posts, working papers, white papers, patents, and more.
Depth and Breadth: Even if an article or book chapter is eventually produced from grey literature, such as a thesis or conference proceeding, the original work may contain data that is never included in the journal article that is ultimately published using its findings. Grey literature might also offer a broader view, such as a government factsheet or institutional newsletter targeted to a lay audience. These can be especially useful when looking for materials to communicate with patients.
Timeliness: Results of studies may appear in grey literature 12 to 18 months before being published via traditional channels.
Flexibility: Rather than waiting years for the publication of a revised edition, authors, editors, and Web content creators can update information when needed, a factor that reinforces the timeliness of grey literature.
Open Access: There is a great abundance of grey literature that is freely available online. Although governments and industries might restrict the readership of certain types of grey literature (classified or proprietary information, for example) there is still quite a bit openly available.
However, grey literature can be hard to discover. Due to the targeted nature of why most grey literature is created it can be hard to index and catalog which increases difficulty of finding it. Utilize the "Searching for Grey Literature" page on this guide to help you navigate through searching.