The dilithium crystal chamber (or dilithium regulation chamber or matter-antimatter chamber or dilithium 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. An engineer on this class of ship could monitor and control the chamber from the Engineering station on the main bridge. (TNG: "Elementary, Dear Data", "Contagion", "Booby Trap", "The Defector", "Galaxy's Child", and more)
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.
There is one type of dilithium chamber that has been shown to date. One was that of the bird-of-Prey HMS Bounty. This was an eight sided chamber with the dilithium crystals inside. The chamber sat on a pedestal in a sealed room. (Star Trek IV: The Voyage Home)