HMD means Head Mount Display
A head-mounted display is a monitor that you carry on your head. A HMD usually has the form of a (big) glasses. The most famous form of a HMD are the many VR glasses that are available for sale today. Despite the ‘ display ‘ in the name, a HMD does not have to have a display: it is also possible to project an image with a laer. The simplest form is an LCD screen with a lens: the lens looks like the display is a few meters away. A HMD can close the environment so that you only see the image, but it can also be transparent and project information into the image. The simplest type of HMD is a holder with lenses where you plug in a smartphone. Https://www.youtube.com/watch?v=0VEX-1iqgDg