Open Educational Resources (OER) are freely accessible teaching, learning, and research materials in any format that are in the public domain or are available under an open license that allows their free use and re-purposing by others.
The term open educational resources (OER) was first introduced at a conference hosted by UNESCO in 2000 and was endorsed in the context of providing free access to educational resources on a global scale.
OER include a varied range of materials: books, case studies, courses, journals, primary sources, assessments, assignments, reference materials, tutorials, tests, and more.
The 5 R's - Source http://opencontent.org/definition/