A three-dimensional projection of light and force fields, used in Star Trek for recreation, training and communication, and able in some cases to become a thinking being.
Holograms become central to Star Trek with the holodeck of the Enterprise-D in The Next Generation. Solid, lifelike characters can be created there, and one of them, Professor Moriarty, becomes aware of his own nature in Elementary, Dear Data.
Star Trek: Voyager makes a hologram a member of the crew. The Emergency Medical Hologram, the Doctor, is activated in Caretaker when the medical staff are killed and grows far beyond his programming, later gaining freedom of movement through a mobile emitter from the 29th century. Deep Space Nine has the self-aware lounge singer Vic Fontaine.
Later series extend the idea. Captain Rios's ship in Picard is staffed by emergency holograms in his own likeness, and in Starfleet Academy the Doctor, some 900 years old, teaches alongside the holographic cadet Sam.