What is User Interface (UI)?

User interface (UI) typically refers to the method of interaction between a user and a program via a digital screen, computer, mobile device, website, or software application. Like a user experience (UX), a proficient user interface should be user-friendly, simple, efficient, secure, and private. User interfaces can also include Human-Machine Interfaces (HMIs) that interact with physical input hardware such as a keyboard, mouse, or gaming controller, or with output hardware such as a computer monitor, speaker, or printer.