The paper uses the surface texture parameter RSm as a case study to examine the concepts of unambiguous definitions of measurands and the stability of these definitions. The paper begins by reviewing the current ISO definition of RSm, illustrating the ambiguities and lack of stability in the current ISO definition. The representational theory of measurement, which is the current dominant paradigm for measurement, will be briefly discussed. The representational theory of measurement will be used as a framework to incorporate a generic definition of stable measurand definitions. Here a measurand is considered mathematically stable if a 'small' difference in the value of a measurand implies a 'small' difference in the property it purports to characterize. This is a completely general and very powerful result. A new stable and unambiguous definition of RSm will be presented which uses these results.