According to the Wikipedia, Open educational resources are:
...freely accessible, openly licensed documents and media that are useful for teaching, learning, and assessing as well as for research purposes. It is the leading trend in distance education/open and distance learning domain as a consequence of the openness movement. Although some people consider the use of an open file format to be an essential characteristic of OER, this is not a universally acknowledged requirement.
One of the most cited definitions of OER is the William and Flora Hewlett Foundation term which defines Open Educational Resources as:
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.
Finally, UNESCO provides the following answer to the question What are Open Educational Resources (OERs)?:
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.