OER stands for "Open Educational Resource." An OER is a piece of course material (such as a textbook, reading, video, slide deck, etc.) that has been made available under an open license. At a minimum, an open license allows you to make copies of the resource and give them to others, both of which are activities not usually allowed under copyright law:
People sometimes make the mistake of referring to any free educational resource as an OER. This is incorrect. Only resources with an open license are OERs. However, there are lots of other types of zero-cost and low-cost resources:
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