A head-mounted display (HMD) is a device worn on the head or as part of a helmet that presents virtual reality or augmented reality content to the user. This type of display typically consists of small screens or lenses positioned in front of the user's eyes, allowing them to see computer-generated images overlaid on their real-world surroundings. HMDs are commonly used in gaming, simulation training, and medical applications, allowing users to interact with and immerse themselves in virtual environments. Research in this area focuses on improving the resolution, field of view, and comfort of HMDs, as well as exploring new applications and technologies for this type of display.