A visual novel is a videogame genre with Japanese roots. It generally has prerendered backgrounds with text, accompanied sometimes by voice acting. Player agency ranges from significant to simply clicking 'next' to read more lovingly rendered text.
Dating sim or visual novel?
If a game is focused on relationships, it is a dating sim. If it isn't, it is a visual novel. Otherwise this all gets WAAAAY too confusing, especially when 3D dating sims which aren't visual novels enter the picture.
Example: Phoenix Wright is not a dating sim.
True Love is a dating sim. One of the better known ones, actually.
Saya no Uta is a not really a dating sim. It is, however, an eroge.
Is that clear?