An antimatter reactor or main reactor was a power generation device that utilized antimatter. Antimatter reactors typically emitted neutrinos as a byproduct.
In 2151, Sub-Commander T'Pol detected neutrino emissions from a city on the Akaali homeworld despite the society being pre-industrial. The neutrinos were later found to be caused by a Malurian antimatter reactor, which was powering a secret underground veridium mining operation. (ENT: "Civilization")
In September 2152, Commander Tucker noted that they had to shut down the main reactor to prevent being broiled inside the catwalk. (ENT: "The Catwalk")