According to UNESCO:
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.
Since 2002, when the term Open Educational Resources (OER) first emerged, to today, OER has increasingly been recognized by the international community as an innovative tool for meeting the challenges of providing lifelong learning opportunities for learners from diverse levels and modes of education worldwide’.
You can view a number of different OER definitions available from the Creative Commons Wiki on their What is OER? page, including:
Open educational resources (OER) are free and openly licensed educational materials that can be used for teaching, learning, research, and other purposes.
Open Education "...is the simple and powerful idea that the world’s knowledge is a public good and that technology in general and the Web in particular provide an extraordinary opportunity for everyone to share, use, and reuse knowledge."
- The William and Flora Hewlett Foundation