Definition of gray literature:
Grey Literature is "That which is produced on all levels of government, academics, business and industry in print and electronic formats, but which is not controlled by commercial publishers moves the field of grey literature beyond established borders into new frontiers, where lines of demarcation between conventional/non-conventional and published/unpublished literature cease to obstruct further development and expansion. At the same time, this new definition challenges commercial publishers to rethink their position on grey literature." Arizona Open Repository
In a nutshell, gray literature are the papers, publications, etc. which usually do not show up in databases or other indexing resources. The following sites provide a more detailed discussion and provide potential resources for searching gray literature. There is a lot of information "out there" so you may wish to discuss this area of research with your professor.
