Nullifier core

From Memory Alpha, the free Star Trek reference.

The nullifier core is a component of a Romulan warp engine. A slight misalignment in one of the nullifier cores of a Romulan warbird can cause the ship's artificial quantum singularity power source to show through the cloak as an intermittent polarized magnetic distortion whenever the ship is moving.

In 2369, the engineer of the IRW Khazara, a sympathizer with the Romulan underground movement, misaligned a nullifier core at Deanna Troi's behest, to signal the USS Enterprise-D that they were nearby. (TNG: "Face of the Enemy")