User interface: the controls, visual elements and feedback through which someone interacts with a product.
A practical example
A form’s labels, fields, error messages and submit button are parts of its interface.
What to watch for
Visual polish is only one part of quality. Check contrast, focus states, error recovery and assistive-technology use.
Related reading
Explore User Experience (UX) and Call To Action (CTA). See how this fits into Web design.