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 or each eye.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 or each eye.