There are many definitions, for example:
OER are educational materials—everything from a single lesson plan to an entire textbook—that save students and teachers money because they are free to use, customize, and share. OER are openly licensed, which makes it easy to personalize materials and infuse them with fresh, relevant content. (OER Commons)
or
Common to these definitions is the idea that OER are not only available at no cost, but that they can be modified, reused, and shared. David Wiley has described the permissions that define an open resource with the "5 Rs:"
Note that OER are not limited to textbooks. They can include any kind of teaching and learning materials from problem sets to complete course designs.
The most common reason driving interest in OER is to reduce the cost of course materials.
But the 5 Rs create many other advantages for OER:
This guide is licensed under a Creative Commons Attribution 4.0 International License.