Open Access (OA) describes online research publications that are free of all access restrictions (ex. pay firewalls) and free of most copyright or licensing restrictions on use. OA can apply to peer reviewed journal articles (and peer reviewed journals), conference papers, theses, book chapters, Ebooks, open textbooks, data sets, learning objects, or media made available through institutional repositories, open educational resource (OER) repositories, etc.
The Right to Research Coalition has published an Open Research Glossary providing all terminology to describe different levels of open-ness, open access formats, licensing, author's rights, and publisher's rights. It is frequently updated to reflect advances in publishing models and technologies.
For open access history, policies, and organizations, please visit Open Access: An Overview.
Open Data is publicly available data structured to allow for full discoverability and use by anyone (Data.gov: Memorandum for the Heads of Executive Departments and Agencies: Definitions).
Open Educational Resources (OER) are any type of educational materials that are in the public domain or introduced with an open license. Anyone can legally and freely copy, use, adapt and re-share them. OERs range from textbooks to curricula, syllabi, lecture notes, assignments, tests, projects, audio, video and animation (UNESCO: What are Open Educational Resources? 2017).
Open Repositories are freely accessible digital collections, usually supported by universities or research institutions, that make research output and creative works available (Open Access Scholarly Information Network, 2009).
Open Textbooks are faculty-reviewed textbooks that are funded, published, and licensed to be freely used, adapted, and distributed (U of M Open Textbook Library).