"Gray Literature" and "Grey Literature" are interchangeable. This guide will use "Gray" unless the specific resource uses "Grey".
The Fourth International Conference on Grey Literature (GL '99) in Washington, DC, in October 1999 defined grey literature as follows: "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."
In general, grey literature publications are non-conventional, fugitive, and sometimes ephemeral publications. They may include, but are not limited to the following types of materials: reports (pre-prints, preliminary progress and advanced reports, technical reports, statistical reports, memoranda, state-of-the art reports, market research reports, etc.), theses, conference proceedings, technical specifications and standards, non-commercial translations, bibliographies, technical and commercial documentation, and official documents not published commercially (primarily government reports and documents) (Alberani, 1990).
SOURCE: greylit.org, retrieved April 2013. http://www.greylit.org/about
Gray literature is documentary material in print and electronic formats, such as reports, preprints, internal documents (memoranda, newsletters, market surveys, etc.), theses and dissertations, conference proceedings, technical specifications and standards, trade literature, etc., not readily available through regular market channels because it was never commercially published/listed or was not widely distributed. Such works pose challenges to libraries in identification (indexing is often limited) and acquisition (availability may be uncertain). Absence of editorial control also raises questions of authenticity and reliability. Alternative methods of supply and bibliographic control have evolved in response to the need to preserve and provide access to such material. In the United States, the gray literature of science and technology is indexed in the NTIS database. Theses and dissertations are indexed and abstracted in Dissertation Abstracts International and are available in hard copy via Dissertation Express. Click here to learn more about finding gray literature, or see the article Gray Literature: Resources for Locating Unpublished Research by Brian S. Mathews in the March 2004 issue of C&RL News. Also spelled grey literature. Compare with ephemera and fugitive material.
SOURCE: Online Dictionary for Library and Information Science by Joan M. Reitz
Retrieved April 2013 from http://www.abc-clio.com/ODLIS/odlis_G.aspx?#grayliterature