Open Educational Resources (OER) is defined by the United Nations Educational, Scientific, and Cultural Organization (UNESCO) as:
..."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."
Source: What are Open Educational Resources (OERs)?