The dilithium crystal chamber (or dilithium regulation chamber or matter-antimatter chamber) was a component in a starship's warp core that contained the dilithium crystals. The chamber included dilithium regulators and antiproton injection seals. Aboard Galaxy-class starships, this chamber was located on Deck 36. (TNG: "Elementary, Dear Data", "Contagion", "Galaxy's Child", "The Drumhead", "Cost of Living")
There are four types of dilithium chamber that have been shown to date. One was that of the NX-class starship, though exactly which component of the reactor was the chamber itself was never disclosed.
The second was used on board the original configuration of the Constitution-class. It was a squat cylinder with a blunt domed top in the center of main engineering. The dilithium crystals were housed in a framework that rose out of one side of the chamber.
The next was the columnar "swirl" chamber, as shown in the Constitution-class refit and the Intrepid-class. Backstage information, including interviews with Rick Sternbach, indicate that in this model of chamber, the entirety of the column is the reaction chamber, which is lined with a layer of dilithium to moderate the reaction.