Is soft focus a technical term or descriptive term?

I say it could be used either way. Years ago, they actually made "soft focus" lenses for portraiture. Imagine, a lens with a "defect" built in on purpose! These days, people used various types of filters to replicate the effect. Therein lies the technical use of the term.

If one describes an image taken in this fashion, I suggest it becomes descriptive in nature.