An artificial quantum singularity is a tiny contained black hole used to power Romulan warbirds in place of a conventional matter-antimatter reactor.
Romulan D'deridex-class warbirds draw their power from an artificial singularity held at the ship's core. The design sets Romulan technology apart from that of the Federation and the Klingons, but it also carries unique risks if containment fails.
The Enterprise-D found the danger firsthand when Picard and others discovered the ship frozen in time during an apparent battle with a warbird. Aliens had been using the warbird's singularity as a nesting site, and their presence created temporal disruptions around the ships.