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 find individual and subject-specific handbooks, dictionaries and encyclopedias. A selective list includes,
MathWorld (current, Wolfram)
Encyclopedia of Mathematics (current, EMS)
European Digital Mathematics Library (current, EuDML)
Handbook of Mathematics (Bronshtein, 2015)
Theses and Dissertations are important unpublished resources granted and retained by universities as capstone projects, these, and dissertations.
Open Education Resources (OERs) are digital ebooks and learning objects that can be used as open textbooks on a wide variety of general and disciplinary subjects.
Current and historical Mathematicians and Statisticians are important resources when exploring ideas, proofs and publications.