Everything displayed on your website that the user can interact with.
The user interface needs to be:
- simple (avoid unnecessary elements)
- clear (use colors, sizes, and layout to create hierarchy and increase scanability)
- consistent (react the same to the user action)
- intuitive (working the way the user expects it to, basically similar to most other websites).
Making a positive impression on our visitor and giving them a good user experience is the purpose of UI. Since these are the factors that affect dwell time and user engagement they are crucial for SEO.