Holodeck reactor was a technology used on Starfleet ships that powered the holodeck.

In 2371, the crew of the USS Voyager tried hooking up the holodeck reactor to the ship's systems for an additional power source. However, it ended up blowing half the relays on the ship. Ensign Harry Kim found that the holodeck reactor was not compatable with Voyager's systems. (VOY: "Parallax")

According to the Star Trek Encyclopedia (4th ed., vol. 1, p. 343), "The writers of Star Trek: Voyager "invented" the holodeck reactors as a way of "explaining" why the ship could have insufficient power to run the replicators, but the crew could still use the holodeck."