Open Educational Resources (OER) are materials in any medium, digital or otherwise, that permit:
OER can include material from the public domain, openly licensed materials or material made available for the above uses through the copyright owner's permission.
-- Based on “The 5 R’s of Openness” http://opencontent.org/blog/archives/3221
and the William and Flora Hewlett Foundation definition.
image: CC BY Sunshine Connelly. Original image