The use of a separate nomenclature for the application of photosensitisers to the oncological and infectious disease fields represents a sensible approach. There is commonality, of course, in that both utilise light activation and act via the local generation of reactive oxygen species, but the difference in cellular targets is so great that different designs are required to achieve proper selectivity for a clinical end use, whether in human or veterinary medicine. The following represents a personal view, and perhaps a clarification of terms, in what might be considered a major etymological dichotomy existing within photodynamic research, on the 20 anniversary of "PACT".