Molecular phase inverter

From Memory Alpha, the free Star Trek reference.

A molecular phase inverter is a device that can alter the structure of normal matter, putting it out of phase with normal matter. Matter that has been "phased" can no longer make physical contact with normal matter. However "phased" matter can make physical contact with "phased" matter that has the same phase variance. Phased matter is also unaffected by external energy sources such as heat, allowing a phased object to be able to, in theory, go inside a planet. In addition, conventional weapons would be completely ineffective against phased matter.

Three different powers, the Klingon Empire, the Romulan Empire and the Federation (with the latter being done illegally) attempted to combine a molecular phase inverter with a cloaking device, creating an interphase cloaking device or an interphase generator (either of which would have provided a substantial tactical advantage), all with limited success. (TNG: "The Next Phase", "The Pegasus"; ENT: "These Are the Voyages...")