The phase cloak was a secret Federation cloaking device that both concealed a ship and shifted it out of phase, letting it pass through solid matter.
Starfleet developed the phase cloak aboard the USS Pegasus in direct violation of the Treaty of Algeron. A young ensign named William Riker served aboard during the testing, under Captain Erik Pressman. A failure during trials led to a mutiny, and the ship was lost and believed destroyed.
Years later the Pegasus was discovered fused inside an asteroid. Pressman, by then an admiral, tried to recover the device, but Riker revealed the experiment and Captain Picard exposed the violation to the Romulans, using the cloak only to escape the asteroid.