In the modern sense of the word, a hologram is a photograph that looks three-dimensional when viewed by the naked eye. However, in the Shye universe, the term has been adopted as a term for any visible three-dimensional light image projected in empty space, including anything from a simple static display to a fully realized simulation of a physical object.
The actual devices used to produce a holographic image are called holographic emitters. Simply put, a holographic emitter is capable of creating tiny pulses of electromagnetic radiation -- i.e., light -- at any point in space within a very limited range. By creating pulses of light in varying positions and in quick succession, a complete three-dimensional image can be produced. However, the resolution a single emitter is capable of is very limited. For this reason, for most serious holographic applications, an array of emitters is normally used. The more emitters that are used to create an image, the higher the resolution of that image can be.