UI
Also known as: User interface
UI is the visual and interactive surface of a product: the layout, typography, colour, and controls a person actually sees and touches.
In full
User interface design covers the presentation layer of a digital product: layout and spacing, typography, colour, iconography, imagery, and the appearance and behaviour of interactive controls such as buttons, inputs, and menus. It is where brand becomes tangible on screen and where usability is either supported or undermined by decisions about hierarchy, contrast, and affordance. UI works in service of UX: a well-designed interface makes the intended path obvious and the correct action the easiest one to take.
Why it matters
Visitors form judgements about credibility within moments of a page appearing, and those judgements are formed visually before any content is read.