PFAS Definition Improvement Act This bill broadens the definition of perfluoroalkyl or polyfluoroalkyl substances, commonly referred to as PFAS, in relation to the reporting requirement under the Toxic Substances Control Act. PFAS are man-made and may have adverse human health effects. A variety of products contain the compounds, such as nonstick cookware or weatherproof clothing. Specifically, the bill provides that PFAS include those substances that contain at least one fully fluorinated carbon atom.