The closest I'm coming to an 18th century term for "classical" music is the "Galant Style." Descriptions of this style make it sound a lot like "classical," but obviously it is not an exact synonym, being somewhat more narrow in its usage. In other words, not all music that we call "classical" (in the narrower sense) is galant, and possibly some galant music we might even call "baroque." Of course, I still don't know what the baroque-era term for baroque was, if any. I suspect I'll find a series of narrower terms for specific styles, some tied to national or regional ideas.