The 2010 International Conference on Grey Literature defines grey literature as follows:
"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 primary activity of the producing body."
To put in another way, grey literature is information that is not published or not typically made available by commercial publishers. There are many different types of grey literature, which includes but is not limited to:
There are several reasons why grey literature should be included as part of any thorough literature search.
1.) There may be little information available on a topic in published scholarly articles
2.) The grey literature may have the most current source of information due to publication lag in peer-reviewed academic journals
3.) The information from grey literature can complement the information from peer-reviewed academic journals, and as a result, provide a more detailed and fuller picture on a research topic