Open Educational Resources (OER) are "teaching, learning, and research resources that reside in the public domain or have been released under an intellectual property license that permits their free use and re-purposing by others."
From The William and Flora Hewlett Foundation
OER materials are released under an open license granting permission for every one to:
OER include digital learning materials such as:
Zero Textbook Cost (ZTC)
ZTC and OER are sometimes confused, but they do not mean the same thing. A ZTC course is one that uses learning materials that cost the students zero dollars, and may include such resources as e-books, videos, and websites. An OER course makes use of Open Educational Resources that are created and published under Creative Commons or other open licensing agreements.