Open Educational Resources (OERs) comprise a wide range of open educational materials (including textbooks, tutorials, lesson plans, simulations, media, entire courses, resources, course materials, lectures, and more) that are specifically created and licensed to be freely used and reused.
Many OERs have been created and openly licensed to grant users full permissions rights, allowing the so-called 4Rs of openness* defined in 2010 by David Wiley, Education Fellow at Creative Commons:
------------- Reuse, Revise, Remix, Redistribute -------------
Some OER advocates have expanded the original framework to include an important 5th R of Retain.
* See D. Wiley, Openness as Catalyst for an Educational Reformation,
EDUCAUSE Review, vol. 45, no. 4 (July/August 2010): 14–20