According to A Dictionary of Public Health Grey Literature is:
Unpublished papers and reports, academic dissertations, etc., that by common agreement are of sufficient quality to be reliable, verifiable, and acceptable sources of original observations for citation purposes.
(2007). gray literature. In Last, J.(Ed.), A Dictionary of Public Health. : Oxford University Press. Retrieved 27 Jun. 2013, from http://www.oxfordreference.com/view/10.1093/acref/9780195160901.001.0001/acref-9780195160901-e-1796.