"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." (from the Fourth International Conference on Grey Literature: New Frontiers in Grey Literature.GreyNet, Grey Literature Network Service. Washington D.C. USA, 4-5 October 1999.)
A summary of the various document types which constitute gray literature is available from GreyNet, an organization dedicated to tracking gray literature.