Open Educational Resources (OER) are educational materials that are licensed to be freely available for teachers and students to use, adapt, share, and reuse.
Examples of OER include learning content (such as lesson plans, assignments, textbooks, exams, and videos) as well as tools for learning (like software, homework systems, interactive diagrams, and more)
Source: Wiley, D. (2014, March 4). The access compromise and the 5th r. [Web log post]. Retrieved from https://opencontent.org/blog/archives/3221