Grey literature is any literature that is not published in the traditional sense. Grey literature is not easily found. It is often excluded from large databases and other mainstream sources. Use grey literature to complement your search.
There are many types of grey literature ranging from scholarly sources to more community-based resources. Examples of grey literature include:
Some library databases contain grey literature in addition to journal articles. For example, Web of Science and Embase contain conference proceedings, and PsycINFO contains dissertations.
Since library databases do not generally provide comprehensive coverage of grey literature, other resources can be searched to identify literature that is not formally published as well.