User Interface
A user interface is the boundary at which a person and a computer system exchange information: everything the system presents for the person to perceive, and every means by which the person can instruct it. A keyboard and a command prompt, a touchscreen with icons, a voice assistant, a car's dashboard and a game controller are all user interfaces, and the term is deliberately broader than any one style, so that the graphical interface, the command-line interface and the terminal are particular kinds of it.
An interface has two directions, and good design closes the loop between them. Output is how the system displays state, such as text, images, sounds or vibration, and input is how the person acts, through keys, pointing devices, touch, gestures or speech. Donald Norman's account in The Design of Everyday Things (1988) describes the gap between what a person wants and the actions available as the gulf of execution, and the gap between what the system did and what the person can perceive as the gulf of evaluation. A good interface keeps both gulfs narrow by offering visible possible actions and prompt, understandable feedback, so that a person always knows what can be done and what just happened.
Interface styles have changed as the available hardware changed. Early computers were operated by switches and punched cards; interactive teletype and then terminal sessions produced the command line; Douglas Engelbart's 1968 demonstration showed the mouse, windows and hypertext, and Xerox PARC and Apple turned these into the graphical interface of the 1980s. The touchscreen smartphone, popularized by the iPhone in 2007, made direct manipulation by finger the dominant style, and voice assistants, augmented-reality headsets and conversational chat interfaces to large language models, which reached mass audiences with ChatGPT in late 2022, are the latest additions. Each style is a trade-off between ease of learning, speed for experts and suitability for the situation, which is why none of them has replaced the others.
Usability is the measurable quality of an interface, and it is more than appearance. The international standard ISO 9241-11 defines it as the extent to which specified users can achieve specified goals with effectiveness, efficiency and satisfaction in a specified context. Practical tools include Jakob Nielsen's ten usability heuristics of 1994, such as visibility of system status, consistency, error prevention and recognition rather than recall, and Fitts's law, published by Paul Fitts in 1954, which predicts that the time to reach a target grows with distance and shrinks with size, the reason screen corners, large buttons and the menu bar at the top of a Macintosh display are fast to hit. Usability is assessed by watching real people attempt real tasks, since designers are poor judges of what feels obvious to others.
The user interface is often confused with its neighbours. The user experience covers the entire relationship with a product, including performance, reliability, documentation, support and emotional response, and the interface is only the part that is directly touched. The interface is also not the application: the same underlying program can offer a graphical front end, a command-line tool and a programming interface, and a well-built system keeps the logic separate from the presentation so that each can change independently. A frequent misconception is that a better interface means a prettier one; in practice, removing an unnecessary step or making an error message say what to do often improves outcomes more than any redesign of colours.
Accessibility and inclusion are now central requirements rather than extras. Interfaces must work for people who cannot see, hear, use a mouse or read small text, and standards such as the Web Content Accessibility Guidelines (WCAG) define concrete criteria, from text alternatives for images to sufficient colour contrast and keyboard operability. Screen readers depend on interfaces exposing a semantic structure and not merely pixels, so accessible design frequently benefits everyone, as with captions in noisy rooms. Modern data and machine-learning work shows the same concerns, since dashboards, notebooks and chat front ends decide whether a model's output is understood, trusted or misused.
Read a user interface as the shared surface where two very different kinds of processors meet: its job is to make what the machine knows visible and what the person wants executable, and every style from the teletype to the touchscreen is a different answer to how that exchange should happen.