“Open Educational Resources are 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.” The William and Flora Hewlett Foundation
In order for educational materials to be considered OER they must be licensed to allow the following:
Creative Commons Licenses are a common way to signal which permissions the creator of an OER grants to users. More information about CC licensing available here.