A hologram was a three-dimensional construction that consisted of photons, forcefields, and matter that was generated by holo-emitters or similar devices. Simple holograms could be of inanimate objects, while more advanced holoprograms could create animated simulacrums of living beings that could interact with real people, and behave as sentient beings.
Applications and uses
Basic hologram applications
The most widespread use of holographic technology in 24th century medicine was the Emergency Medical Hologram, or EMH. EMHs such as the Mark I through IV were all designed by famed hologram designer Dr. Lewis Zimmerman.
Holographic technology has also been used as a stopgap measure to replace patients' damaged or missing internal organs. This method was first used by the EMH Mark I aboard the USS Voyager in 2371, when passenger Neelix had his lungs removed by Vidiians. (VOY episode: "Phage") Dr. Elizabeth Lense used this same method in 2377, after Crewman Bart Faulwell had his kidney, pancreas and spleen damaged by a projectile. (CoE eBook: Signs from Heaven)
In the 31st century, almost any living being can have their minds uploaded into a mobile emitter via hologram technology. The upload process is said to be quite painless and can extend that life form's existence almost indefinitely. (DTI eBook: The Collectors)