The term 'grey literature' refers to a wide range of information which is not formally or commercially published.
Grey literature is usually produced by government departments and agencies, local authorities, academic institutions, professional organisations, research groups, think tanks, charities and comes from many other sources.
Examples of grey literature include the following types of materials: reports, policies, briefings, pamphlets, conference proceedings, clinical trial registries, doctoral theses, interviews, social media outputs (such as blogs, podcasts, etc.).
If you would like to explore further, below are a few articles which give examples of the debate about and the use of grey literature in research.