The terms "open content" and "open educational resources" describe any copyrightable work (traditionally excluding software, which is described by other terms like "open source") that is licensed in a manner that provides users with free and perpetual permission to engage in the 5R activities:
Source: Open Content definition by David Wiley
Note: reused and adapted from University of Texas Arlington Libraries, Open Educational Resources by College Guide.
Examples of what can be used as OERs:
Note: from Northwest Michigan College: Michigan Community College Librarians OER guide.