Grey literature is literature produced by any organisation whose central purpose is not publishing. These organisations include governments, businesses, not-for-profits, health organisations and associations, and more. Searching for grey literature can be difficult because it is not usually indexed within traditional databases. When completing a systematic review, however, it is imperative to find as much evidence as possible, and that means searching beyond academic literature alone.
See: Hartling L, Featherstone R, Nuspl M, Shave K, Dryden DM, Vandermeer B. Grey literature in systematic reviews: a cross-sectional study of the contribution of non-English reports, unpublished studies and dissertations to the results of meta-analyses in child-relevant reviews. BMC Medical Research Methodology. 2017 Apr 19;17(1):64. http://dx.doi.org/10.1186/s12874-017-0347-z
Scopus and Web of Science include conference proceedings in their listings.