The Fourth International Conference on Grey Literature (GL '99) in Washington, DC, in October 1999 defined grey literature as follows: "That which is produced on all levels of government, academics, business and industry in print and electronic formats, but which is not controlled by commercial publishers."
In general, grey literature publications are non-conventional, fugitive, and sometimes ephemeral publications. They may include, but are not limited to the following types of materials: reports (pre-prints, preliminary progress and advanced reports, technical reports, statistical reports, memoranda, state-of-the art reports, market research reports, etc.), theses, conference proceedings, technical specifications and standards, non-commercial translations, bibliographies, technical and commercial documentation, and official documents not published commercially (primarily government reports and documents) (Alberani, 1990).
Source: What is Grey Literature? New York Academy of Medicine http://www.greylit.org/about
You can select your search preferences in Google Scholar in order to retrieve Grey Literature. Check out this tutorial from the Lister Library at the University of Alabama at Birmingham.
The "Invisible College" is where faculty, researchers, and other experts in a particular field exchange ideas that are not published nor made widely available. Email, blogs, e-news, listservs, chat, and other such sources make up the invisible college. The information may be brief, but usually current. In some cases, this information will be impossible to find. However, blogs, e-conferences, or listservs can be tools to use for discovering some of these elusive sources.
Source: Information Literacy @ Old Dominion University Web Page
Here is an interesting article about Invisible Colleges:
Carey, John. (2011). Faculty of 1000 and VIVO: Invisible Colleges and Team Science. Issues in Science and Technology Librarianship, Issue 65.
Special thanks to John Carey, Head Librarian, Health Professions Library, Hunter College City University of New York for permission to link to his article.