OER stands for Open Educational Resources and it is a digital as well as non-digital educational materials that can be used, copied and distributed free of charge and without permission from the creator. In some instances, It can be also be adapted to suit different contexts and uses. -William and Flora Hewlett Foundation
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. -UNESCO
For more definitions of OER, see the Creative Commons Wiki.