It is sometimes defined by what it is not. Soft tissue has been defined as "nonepithelial, extraskeletal mesenchyme exclusive of the reticuloendothelialsystem and glia".[2]"
Stroma is generally non-parenchymal tissue within an organ that plays a structural role. Parenchyma is the functional tissue of an organ. So if you take an organ's non-functional tissue, you could basically call that stroma.
Since stroma plays a structural role (i.e. is supportive just like soft tissue) I'm going to equalize them as terms as the term is seldom used in the literature (FA/Pathoma).