Note regarding V1.2: there are many papers which take roughly V1.1, but without the 'large' and the 'no definite shape' to be not equating some object and not equating with a quantity of the matter. However, neither of the papers provide a definition and instead illustrate the difference with examples and characteristic features as distinct from objects and count nouns.
In short: the term is polysemous and can refer to either the stuff or an amount of it, depending on the context the word is used.
RR: why is 'mass' a term? It seems like it is a physics-specific term.