UI

In short: UI (user interface) is everything users interact with in a system: screen, buttons, menus, voice.

In more detail: It is the visible layer. Good UI is clear, consistent and accessible.

In Depth

Components

Layout, typography, colours, buttons, forms, feedback. See GUI for graphical interfaces and UX for the overall experience.

Accessibility

Sufficient contrast, keyboard operation and alternative texts are part of it (see WCAG and colour contrast).

See also: UX, GUI, Frontend, WCAG und Farbkontrast