Science fiction writers have used the name "holocube" to denote three related, yet quite different, types of futuristic imaginary technology.
The first, and most common, type of holocube is a three-dimensional holographic projection device. Essentially, a holocube is a cube of transparent or translucent material, or even simply an empty cubical space, within which a holographic projection, either a recording or a transmission, appears. In this vision of the future, people don't watch flat-screen televisions, they watch "tri-v" or "3V channels" on holocubes. This idea can be found in the works of Brian W. Aldiss, amongst others, such as Supertoys Last All Summer Long: And Other Stories of Future Time and The Eighty-minute Hour: A Space Opera. Similarly, holocubes, in which a full three-dimensional image of one's interlocutor appears, replace the humble telephone.
The second most common, but obviously much smaller, type of holocube is simply a cube of some unspecified material, that is used as a holographic storage device. This is simply the classic "data crystal" motif.
A third, and less common still, type of holocube is simply a three-dimensional photograph. In David Weber's Honor Among Enemies, for example, the eponymous Honor Harrington carries around a holocube containing a three-dimensional image of Paul Tankersley.