According to the William and Flora Hewlett Foundation, Open Educational Resources are "high-quality teaching, learning, and research materials that are free for people everywhere to use and repurpose."
According to UNESCO, Open Educational Resources (OERs) are any type of educational materials that are in the public domain or introduced with an open license. The nature of these open materials means that anyone can legally and freely copy, use, adapt, and re-share them. OER range from textbooks to curricula, syllabi, lecture notes, assignments, tests, projects, audio, video, and animation.
For more information on what is and is not OER see this short learning module from the Michigan chapter of the American Library Association. https://certificate.mioer.org/module-1/what-are-open-educational-resources