Grey literature databases identify resources that are key to research and scholarship. Many are open resources that go through a quality control process before they are published.
Definitions are scholarly dictionaries, encyclopedias and handbooks that help define terms and provide additional context. Search Omni to discover many different handbooks, reference and data resources. Here is a short selective list,
CRC Handbook of Chemistry & Physics (2014/2015)
Dictionary of Physics (2019)
Weisstein"s World of Physics (current, Wolfram)
Patents are legal documents issued by a national or international patent agency that document an intellectual property to an exclusive invention, product or process
Standards are important documents that define a set of technical definitions, specifications, and guidelines for a product, process, or guideline.
Occupational Health and Safety information is essential to dealing with injuries, accidents, government and regulatory information.
Theses and Dissertations are important unpublished resources granted and retained by universities as capstone projects, these, and dissertations.
Open Education Resources are digital ebooks and learning objects that can be used as open textbooks on a wide variety of general and disciplinary subjects.