The Hewlett Foundation defines 

"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 open movement promotes the use of these textbooks and other materials for free or at a minimal charge. By collaborating to create high-quality shared texts, the academic community can dramatically lower the cost of textbooks and class materials for the students they teach.