Grey literature

The documents published for a limited audience, outside the major distribution channels, and difficult to find in the usual databases, such as presentations at conferences, health technology assessments done by hospitals, and certain government documents.

Note: There is a lack of unanimity on the definition of grey literature, as the boundaries of this literature continually shift and evolve as new technical means are developed.

Alternative spelling: gray literature

Reference

http://htaglossary.net/grey-literature