The holomatrix was the programming code for holographic software such as holonovels, the Emergency Medical Hologram, and other holoprograms. (TNG; DS9; VOY)
Holomatrices could, by its nature, hold more information than standard transmissions of the same size. (VOY: "Inside Man")
It was also a term used to refer to the hologrid, the systems used to project holographic images on the holodeck.
By the 29th century, the term holomatrix was used to refer to that century's equivalent to a holodeck. Braxton had Seven of Nine report to Holomatrix 1 aboard the USS Relativity to run a simulation of her mission aboard USS Voyager to remove a temporal disruptor. (VOY: "Relativity")