A head-mounted display or helmet-mounted display, both abbreviated HMD, is a display device, worn on the head or as part of a helmet, that has a small display optic in front of one (monocular HMD) or each eye (binocular HMD). There is also an optical head-mounted display (OHMD), which is a wearable display that has the capability of reflecting projected images as well as allowing the user to see through it.