Short for head-mounted display, a headset used with virtual reality systems. An HMD can be a pair of goggles or a full helmet. In front of each eye is a tiny monitor. Because there are two monitors, images appear as three-dimensional. In addition, most HMDs include a head tracker so that the system can respond to head movements. For example, if you move your head left, the images in the monitors will change to make it seem as if you're actually looking at a different part of the virtual reality.