A head-mounted display (HMD) refers to a wearable device that is mounted on a user's head and displays virtual reality (VR) or augmented reality (AR) content. There are several synonyms for HMD, including VR headset, AR headset, smart glasses, wearable display, and head-up display (HUD). VR headsets immerse users in a completely virtual environment, while AR headsets superimpose digital images onto the real-world environment. Smart glasses are similar to AR headsets but tend to be lighter and less bulky. HUDs display information in the user's field of view, usually in the form of a transparent screen that does not block the real-world view.