How Would we Recognize a Martensitic Transformation if it Bumped into us on a Dark & Austy Night ?

The history of the meaning of "martensitic transformation" over the past forty years is briefly reviewed, and an area of common agreement is identified centered on the concepts of "cooperative atomic motion" and "shape change". A search is made for the minimum elements required in a definition that will be simple, compact, precise and easily tested. It is argued that cooperative atomic motion implies acoustic emission, and that this could provide an important operational element in the definition. A distillation of past definitions, with this new ingredient added yields, as a proposed irreducible definition : A martensitic transformation involves a cooperative motion of a set of atoms across an interface causing a shape change and sound.