The term grey literature is used to describe a wide range of different information that is produced outside of traditional publishing and distribution channels, and which is often not well represented in indexing databases.
A widely accepted definition in the scholarly community for grey literature is ...
"information produced on all levels of government, academia, business and industry in electronic and print formats not controlled by commercial publishing" ie. where publishing is not the primary activity of the producing body."
From: Third International Conference on Grey Literature in 1997 (ICGL Luxembourg definition, 1997 - Expanded in New York, 2004).
Uniqueness and currency v quality and longevity
Grey Literature can be a very important research resource.
You should be aware of the need to assess and capture potentially useful resources
If you prefer an audiovisual introduction, take a look at this short information video.
A wide, and growing, range of material can be considered as grey literature.
Not all these examples will be relevant to all researchers. For example, clinical trial information is primarily of interest to health and medical research. You should consider which types of information you are interested in before you begin searching as this will help you target and frame your online searching.