"OER are teaching, learning, and research resources that reside in the public domain or have been released under an intellectual property license that permits their free use and re-purposing by others. Open educational resources include full courses, course materials, modules, textbooks, streaming videos, tests, software, and any other tools, materials, or techniques used to support access to knowledge." 
Open Textbook Library - An open textbook referatory that links to more than 1,000 open textbooks as of the spring 2022 semester.
Mason OER Metafinder - MOM is a real-time OER search tool that searches 22 unique OER sources. Due to the variability of metadata in the sources, some search results may not be openly licensed.
OpenStax - An educational initiative from Rice University that publishes peer-reviewed, openly licensed college textbooks that are absolutely free online and low-cost in print. Open textbooks in the OpenStax collection are focused on the highest enrollment courses that have traditionally used high-cost commercially produced course materials.
The terms "open content" and "open educational resources" describe any copyrightable work (traditionally excluding software, which is described by other terms like "open source") that is licensed in a manner that provides users with free and perpetual permission to engage in the 5R activities:
Save students and taxpayers money: