The currently omnipresent term for someone involved in designing, prototyping, testing and refining the usability of online and desktop interfaces is "UX/UI designer".
Alas, that term is nothing but a fancy and pretentious way to refer to a visual designer. Take a look at random "UX/UI designer" job description on a job board and you'd see requirements for graphic design competences such as the Adobe suite of products and portfolio of visual work. Sometimes "research, prototypes, wireframes" are mentioned, but that's all there's to them. No one is looking for an actual UI portfolio covering categorization/ information architecture, plans for moderated usability tests, recordings of such tests, reports how findings from such tests have been interpreted and applied, specific UI challenges and their solutions (e.g. how shall we display vast spreadsheets on mobile phone screen), etc.
Is there a widespread term denoting the role of doing the actual usability work?
(Nielsen use the term "usability professional" but that is by no means mainstream. I'm wondering, maybe there's no proper widespread term, because there's no widespread usability job position on the first place? Companies seem to expect visual designers to do the usability work and perceive usability as an extension of visual design competencies.)