An interphase device was a type of cloaking technology.
During the 24th century, the Klingon Empire developed an experimental interphase generator by combining a cloaking device with a molecular phase inverter, which could change the structure of matter to pass through normal matter and energy, it would give a vessel the ability to hide anywhere, even inside a planet. Additionally, such a "phased" vessel would be immune to conventional weapons. The Klingons eventually abandoned this research due to several accidents which resulted from these experiments.
In 2368, the Romulans attempted to develop their own interphase generator and install it aboard a science ship, but inadvertently caused a forced chamber explosion in their resonator coil, severely damaging the ship. The USS Enterprise-D answered the Romulans' distress call, and assisted them in making repairs. Upon beaming back to the Enterprise, however, Lieutenant Commander Geordi La Forge and Ensign Ro Laren became contaminated with chronitons leaking from the Romulans' cloaking device, and became phased themselves, unable to interact or communicate with anyone but each other. La Forge and Ro were eventually de-phased when they were subjected to a high-intensity anyon emission. (TNG: "The Next Phase")
In 2373, the Voth used a form of interphase as a cloaking technology on themselves and their ships. This allowed Forra Gegen and Veer to covertly observe the USS Voyager and her crew. The technology was more advanced than anything Tuvok had seen before. When cloaked, the Voth only appeared as localized spatial fluctuations with a modulating phase variance that could be detected with sensors. When the two Voth were isolated from their subspace frequency by analyzing the scans of the spatial phase variance, it was possible to disrupt their cloaking devices with a hand phaser set for a dispersion frequency of 1.85 gigahertz. (VOY: "Distant Origin")