Artificial quantum singularity

From Memory Alpha, the free Star Trek reference.

The artificial quantum singularity (also referred to as a confined or forced quantum singularity) is a method of generating energy .

In the 24th century, artificial quantum singularities served as the primary power source for large Romulan spacecraft, such as the D'deridex-class warbird. Once activated, the singularity core cannot be shut down. (TNG: "Face of the Enemy", "Timescape"; DS9: "Visionary")

In the Delta Quadrant, a network of ancient relay stations claimed by the Hirogen were powered by quantum singularities approximately a centimeter in diameter, putting out four terawatts of energy. (VOY: "Hunters")