the.com/user interface

means The point of contact where a human and a machine exchange input and output — screens, buttons, menus, and gestures that let you tell a computer what to do and let it show you what happened.

from A 20th-century coinage built from two separate borrowings: 'user' from Latin 'uti' (to use), and 'interface' — a word originally coined in physics and chemistry for the boundary surface between two substances, like where oil meets water. As computing grew, 'interface' was borrowed into engineering for the point where two systems meet and exchange signals, and by the mid-to-late 1900s, 'user interface' emerged specifically to name the boundary between human and machine — the place where our intentions get translated into something a computer can act on.