UNESCO defines Open Educational Resources (OER) as “teaching, learning and research materials in any medium – digital or otherwise – that reside in the public domain or have been released under an open license that permits no-cost access, use, adaptation and redistribution by others with no or limited restrictions."
Key to this definition is that OER are educational materials ("teaching, learning, and research materials") that are free ("no-cost') for anyone to use and change ("access, use, adaptation and redistribution by others with no or limited restrictions"), which can be done because they are made open ("reside in the public domain or have been released under an open license").
