A matter/antimatter reaction assembly (often known simply as an M/ARA) is a type of energy reactor which is fueled by injectors of matter and antimatter into a central reaction element, creating large amounts of energy when those two materials mutually annihilate each other. Matter and antimatter is injected into the M/ARA at an intermix ratio. (TNG: "Coming of Age") The dilithium crystal is used to moderate this reaction.
The prototype Jem'Hadar battleship had a fatal design flaw; the braces holding together the ship's M/ARA were vulnerable to a specific type of radiation. The crew of the USS Valiant believed they could use this flaw to their advantage when they were plotting the ship's destruction in 2374. (DS9: "Valiant")