"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. OERs range from textbooks to curricula, syllabi, lecture notes, assignments, tests, projects, audio, video and animation." - UNESCO
As the definition above illustrates, OER are not limited to only open textbooks; however, textbook affordability is a central issue for many OER initiatives.
If you'd like to learn more about OER and textbook affordability, visit the Open Educational Resources and Affordable Textbooks guide.