A power matrix (or energy matrix) is a term for the main energy generation system for a starship or other technology.
Shortly after the USS Voyager was stranded in the Delta Quadrant in 2371, the crew attempted to supplement the ship's energy reserves from the holodeck. However, they found that the holodeck's energy matrix was not compatible with the main power grid. (VOY: "Parallax")
In 2372, B'Elanna Torres modified a series of anodyne relays to make Voyager's warp plasma compatible with the energy matrix of Pralor Automated Unit 3947. (VOY: "Prototype")
In 2374, Seven of Nine designed an energy matrix that would produce thorium isotopes for the Caatati. The Voyager crew also gave specifications and components to build more, allowing them to become self-sufficient. (VOY: "Day of Honor")
In 2375, the USS Voyager beamed a photon torpedo aboard an attacking Borg probe. The torpedo detonated near the probe's power matrix, causing a chain reaction that destroyed the vessel. (VOY: "Dark Frontier")
Later that year, Lieutenant Torres reinitialized the power matrix aboard a damaged Malon freighter. However, only one containment grid was operational, which was insufficient to stabilize both antimatter waste tanks. (VOY: "Juggernaut")
The Druoda series 5 long-range tactical armor unit is equipped with an advanced, extremely dense power matrix, which the merchant Onquanii claimed was capable of powering a fleet of starships. It was also worth a thousand dilithium crystals. When such an armor unit took over Voyager's EMH in late 2375, Onquanii refused to help the crew remove the weapon unless he were given the power matrix. He subsequently attempted to take it by force, resulting in the weapon destroying his ship. (VOY: "Warhead")
In 2377, Voyager disabled a Lokirrim warship by firing phasers at its power matrix. (VOY: "Body and Soul")