The Zero-Point Module is an Ancient power source, used to power their cities and outposts using zero-point energy. It is more powerful and efficient than any energy source on Earth, and can last for thousands of years. While the energy output is huge - significantly larger than that of Naqahdah generators - it appears that the ZPM is based on an irreversible thermodynamic process and will ultimately reach its maximum entropy limit, at which point it becomes practically useless (apart from to throw at annoying people).The Republic has managed to obtain "many" of these, but have refused to speculate where from or how many they have.
Zero Point Modules are used to great affect by the Republic, with Firebirds using them as their primary power source. Also, ships such as Brace and the Scmitar have tens of Zero point Modules powering themselves and their weapons. When s-t-n visited earth after the covenant attack, it is believed he left A/Some zero point moules behind because scans before and after the ship landed on Earth showed a massive decrease in energy within the cargohold.
The Asuran replicators are able to manufacture Zero Point Modules, to power their ships and cities. How they do this is unknown, but it is not suprising they can as they have attempted to imitate their creators (the ancients) in nearly every way.
