Matter-antimatter annihilation
Matter-antimatter annihilation is a theoretical form of power generation. The system brings particles of matter into contact with their oppositely charged anti-particles, allowing them to annihilate each other and release a burst of radiation. The system then converts the radiated energy into work.
The theoretical limit of M/AM annihilation power is defined by Einstein's classic equation of matter-energy equivalency: E=Mc2.
An obvious problem with M/AM annihilation power systems is the inherent difficulty of obtaining, containing, and transporting substantial quantities of antimatter.
In Science Fiction
Matter-antimatter annihilation is a common means of generating power in science fiction, especially Star Trek. The M-AM reactor used in Star Trek is called a warp core.