User Interface (UI)
The design of the specific interactive elements through which a person engages with a digital system.
In practice
User interface design covers the specific screens and controls a person uses in a digital product: buttons, menus, fields, icons, type and layout, including how each looks in states such as pressed, disabled or loading. Clear UI makes actions obvious and consistent. A banking app's transfer screen, with its amount field, recipient picker and confirm button, is a piece of UI design.
Often confused with User Experience (UX)
UI is the specific interactive elements on screen; UX is the person's entire experience with the product or service, of which the UI is one part.
Try this
Screenshot one app screen and label each element, noting its normal, pressed, disabled and error states.