While there is no one, standard definition of Open Educational Resources, UNESCO offers this:
"Open Educational Resources (OER) 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."
OERs are typically:
Open Textbooks are a type of OER that are designed to be a free or low cost alternative for a traditional textbook.
Why Open Textbooks?