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."
In short, they refer to information or data that has been collected or curated by libraries or repositories, but which have not been published as an academic article or book.
You should also try to explore the below other sources of grey literature:
Grey literature can be of high quality and accuracy, and may even be the basis on which many research studies are done. Depending on the type of grey literature, it can be even more current and up-to-date than published academic papers, and can be a source of useful, high quality data.
The significance of grey literature to writing a literature review cannot be overstated. To give a balanced view and overcome publication bias (a phenomenon where there is a disproportionately smaller amount of research published about experiments that have failed and the potential reasons why), drawing upon grey literature is very important.
Do note that it is very important to properly evaluate information from grey literature, however. As these refer to sources of information that have not gone through the commercial publishing process, it is important to ensure that the data described is accurate by cross-checking them with other sources of information before using them.