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 i.e., where publishing is not the primary activity of the producing body."1
This includes data from many different sources with varying levels of reliability and review including government documents, dissertations, conference proceedings, newsletters, and blogs. A major advantage of grey literature is that because information is not published through commercial publishing channels (e.g., peer-reviewed journal articles) the information can be published quicker. This is especially important with emerging issues.
However, the quality of grey literature will have wide variability. For example, one would expect a dissertation, which has undergone numerous reviews before a doctoral student can graduate, to be of a higher quality than a newsletter, which has little to no peer review. However, both dissertations and newsletters are considered grey literature.2
1. Third International Conference on Grey Literature in 1997 (ICGL Luxembourg definition, 1997 - Expanded in New York, 2004).
2. "Libguides: Grey Literature: What Is Grey Literature?" University of Exeter Library, https://libguides.exeter.ac.uk/c.php?g=670055&p=4756572.