A hologram is a three-dimensional image projected onto a two-dimensional surface by carefully controlling the diffraction or reflection of light to produce an illusion of depth.
Holograms in Science Fiction
In science fiction stories, holograms can typically be projected into empty air instead of onto a flat surface.
Holograms in Star Trek
In Star Trek, holograms are used primarily in entertainment systems known as holodecks. In these systems, light is controlled to produce three-dimensional images on walls and in free space within the holodeck chamber. Force fields and replicator technology are used to give solidity to the images.
Holograms in Star Wars
In Star Wars, holograms are primarily used in communication systems to allow "face to face" conversations over interstellar distances. They are also used in military command-and-control centers to provide leaders with three-dimensional views of a combat zone.