Grey Literature is defined as "information produced on all levels of government, academia, business and industry in electronic and print formats not controlled by commercial publishing." ICGL Luxembourg definition, 1997, expanded in NY, 2004
Grey literature is not formally part of "traditional publishing cycles. Producers include research groups, non-profits, universities and government agencies.
Grey literature is not widely disseminated - dissemination of published materials is the goals in traditional publishing. Often, infrastructure exists to disseminate this material efficiently.
"Grey Area" or Grey Zone", think of grey zone as an "in-between" metaphor.