The Research Excellence Framework (REF) is an assessment that is carried out periodically to determine the quality of research in UK higher education institutions. The results are used by the higher education funding bodies for England, Scotland, Wales and Northern Ireland to allocate research funding. The REF replaced the Research Assessment Exercise (RAE) and the outcomes of the first REF were completed in December 2014.
The documentation and the institutional guidance on REF 2014 is still available here.