Facial Action Unit

A component of the Facial Action Coding System (FACS) that represents a specific movement or position of facial muscles. Each action unit corresponds to a distinct facial movement, such as raising the inner eyebrow (AU1) or pulling the lip corner (AU12 for smiling). In accessibility applications, facial action units enable hands-free computer input for people with motor impairments who retain control of their facial muscles. Software like OpenFace can detect action units from webcam video in real-time, allowing facial expressions to be mapped to commands such as mouse clicks, keyboard input, or device control.