OER are digitized materials offered freely and openly for educators, students and self-learners to use and re-use for teaching, learning and research. OER include:
◾Learning Content: full courses content modules, learning objects, collections and journals.
◾Tools: software to support the development, use, re-use and delivery of learning content including searching and organization of content, content and learning managements systems, content development tools, and on-line learning communities.
◾Implementation resources: intellectual property licenses to promote open publishing of materials, design principles of best practices, and localisation of content”
Source: EU Commission LLP 2007-2013, Europe

Open Educational Resources (OER) are teaching, learning and research materials in any medium – digital or otherwise – that reside in the public domain or have been released under an open license that permits no-cost access, use, adaptation and redistribution by others with no or limited restrictions.
OER form part of ‘Open Solutions’, alongside Free and Open Source software (FOSS), Open Access (OA), Open Data (OD) and crowdsourcing platforms.
Source: UNESCO 2016, Global

Open Education Resources (OER) is defined by the OER Paris Declaration 2012 as any "teaching, learning and research materials in any medium, digital or otherwise, that reside in the public domain or have been released under an open license that permits no-cost access, use, adaptation and redistribution by others with no or limited restrictions. Open licensing is built within the existing framework of intellectual property rights as defined by relevant international conventions and respects the authorship of the work".
Source: COL 2015, Global