Star Trek term

Acronym for: Matter/Antimatter Reaction Assembly

As the warp propulsion system is the heart of most Federation vessels, the matter/antimatter reaction assembly (M/ARA) is the heart of the warp propulsion system. The M/ARA is variously called the warp reactor, warp engine core or main engine core. Energy produced within the core is shared between its primary application, the propulsion of the starship, and the raw power requirements of other major ship systems. The M/ARA is the principal power-generating system because of the 10^6 times greater energy output of the matter/antimatter reaction over that of standard fusion, as found in the impulse propulsion system.

The M/ARA consists of four subsystems:
Reactant injectors
Magnetic constriction segments
Matter/Antimatter Reaction Chamber (M/ARC)
Power transfer conduits

Source: Star Trek: The Next Generation Technical Manual


Personal commentary:

While the whole concept behind warp field mechanics is still in its infancy in the real world, there are still some very interesting studies done on it. A well-known Trek anecdote is that Stephen Hawking, when he made a cameo appearance on TNG (Descent: Part 1), he was given a tour of the behind-the-scenes sets. When they brought him into the Engineering Room of the Enterprise-D, they decided to give him the full show. All the bells and whistles were running, as though the place were truly functional as an M/ARA. Hawking just smiled knowingly at everyone, nodded at the M/ARC (which is the center-piece of the Engineering Section that we all see whenever they're looking at the "engines") and then his vocoder announced, "We're working on that, you know." Reportedly, the people present who heard him say this didn't know if they should laugh or fear for his sanity.

Short description of what happens in the M/ARA: The Matter/Antimatter Reaction Chamber is where all the real fun stuff happens with warp engines. In a nutshell, matter and antimatter are injected into the chamber by way of the Reactant Injectors, as electromagnetically compressed streams, where they both collide on the focus of a dilithium crystal. As soon as they meet at the crystal's tip, a finely-tuned beam of plasma energy is ejected from the dilithium crystal (which acts as a sort of energy sponge when the matter and antimatter react explosively on its tip). The tuned plasma is then directed into the Power Transfer Conduits and from there to the Warp Field Nacelles, where the energy from the M/ARA is effectively turned into warp fields.