This report summarises a review of the universe and terminology of per- and polyfluoroalkyl substances (PFASs) to provide recommendations and practical guidance to all stakeholders with regard to the terminology of PFASs. In particular, this report highlights a revised PFAS definition to comprehensively reflect the universe of PFASs, practical guidance on how to use the PFAS terminology, a systematic approach to characterization of PFASs based on molecular structural traits to assist stakeholders, including non-experts, in making their own categorisation based on their needs, and areas in relation to the PFAS terminology that warrant further development.