Perfluoroalkyl substances (PFAS) represent a broad group of synthetic chemicals that have raised concerns related to their long-term environmental persistence and potential health risks. Although several efforts have been dedicated to establishing international restrictions on their use, the definition of what qualifies as a PFAS remains a matter of debate among scientists, regulatory agencies, and industry. This article provides a brief overview of the different approaches proposed and adopted to date for identifying and grouping of these pollutants, either based on common structural motifs or on the combination of multiple factors, including functional uses, degradation behavior, physicochemical properties, and toxicity. The diversity and complexity of PFAS substances suggests the need of a multifaceted classification system that can guide regulatory efforts, risk assessment, and environmental monitoring through standardized criteria accepted on an international scale. A pivotal role in establishing a universal definition of PFAS will be played by the International Union of Pure and Applied Chemistry (IUPAC), which is currently supporting a project on the terminology and classification of these chemicals.