A hologram watch is a watch that projects the time into the air rather than onto the face of the watch. There are several different designs of hologram watches, but one of the most popular designs looks like a bracelet that doesn't make a complete circle. On these wrist watches, the time is projected into the empty space between the two sides of the bracelet.
Another popular hologram watch design features a classic watch face with a missing center. The hologram is projected into that center. Some smartphones can also create a hologram expressing the time.
A hologram is a three-dimensional image that is created by artfully diffracting lasers in certain ways. When someone looks at an hologram, they see light that has been scattered from multiple points in a very particular way to create a lifelike image. In comparison, the light used to create a photograph only comes from a single source.
Holograms are not just used for watches. Holograms of dead celebrities like Michael Jackson have been designed to give fans a posthumous show, and holograms of bank tellers allow bank customers to feel like they are dealing with an actual teller even though they are simply using the cash machine.