There are many definitions, for example:
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: