The workplace (industrial air hygiene) standards and community air pollution regulations implemented by OSHA and EPA, respectively, typically incorporate physical descriptions of the particulate matter (PM) that include particle size. Terms like "respirable", "thoracic", and "inhalable" are used to describe the concentration limits defined by the regulations or standards. More recently, EPA has promulgated new National Ambient Air Quality Standards (NAAQS) using terms such as "fine" (PM_2.5) and "coarse" (PM_10-2.5) for PM. Concentration limits defined by the OSHA and EPA standards and regulations are specified in terms of mass per "standard volume" rather than mass per "actual volume". Specially designed samplers are used to measure the concentrations to determine compliance. These samplers generally consist of two stages. The first stage separates the PM larger than the size specified by the regulation or standard allowing the PM of interest to be measured on a filter in the second stage. The separating efficiency of the first stage is not 100%. As a consequence, a fraction of the PM larger than the particle size being regulated penetrates to the filter and a fraction of the PM smaller than the size being regulated is deposited in the first stage. When describing particle size, the term "aerodynamic equivalent diameter" (AED) is used. This paper includes discussions of engineering terms and descriptors and eliminate some confusion.