A holodeck reactor was reactor technology used on Starfleet ships that powered the holodeck.
In 2371, the crew of the USS Voyager tried hooking up the holodeck reactors to the ship's systems as an alternative energy source. However, the holodeck reactors being hooked to the power grid had ended up blowing half the relays on the ship. Ensign Harry Kim found that the holodeck's energy matrix was not compatible with Voyager's other power 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."