stylistic:tone, information:classification
Terminology

There is a difference between a user interface and an operator interface or a human–machine interface (HMI).

In science fiction, HMI is sometimes used to refer to what is better described as a direct neural interface. However, this latter usage is seeing increasing application in the real-life use of (medical) prostheses —the artificial extension that replaces a missing body part (e.g., cochlear implants).

In some circumstances, computers might observe the user and react according to their actions without specific commands. A means of tracking parts of the body is required, and sensors noting the position of the head, direction of gaze and so on have been used experimentally. This is particularly relevant to immersive interfaces.