OER are publically accessible teaching, learning, and research resources created and licensed to be free for end users to own, share, and in some cases modify.  Modify could include re-mixing, improving, and redistributing under some licenses.

UNESCO's 2019 definition: "Learning, teaching and research materials in any format and medium that reside in the public domain or are under copyright that have been released under an open license, that permit no-cost access, re-use, re-purpose, adaptation and redistribution by others. Stakeholders in the formal, non-formal and informal sectors (where appropriate) include: teachers, educators, learners, governmental bodies, parents, educational providers and institutions, education support personnel, teacher trainers, educational policy makers, cultural institutions (such as libraries, archives and museums) and their users, information and communications technology (ICT) infrastructure providers, researchers, research institutions, civil society organizations (including professional and student associations), publishers, the public and private sectors, intergovernmental organizations, copyright holders and authors, media and broadcasting groups and funding bodies."