How it is to be the person using a thing. UX is the whole felt experience, of which the visible surface is only part.
A late-20th-century computing initialism joining user (Latin uti, to use) and experience (Latin experientia, a trial, from experiri, to try), naming the whole felt encounter beyond the visible surface.
The whole felt experience of using a thing, of which the visible surface a person interacts with is only one part.