A phase discriminator
A phase discriminator was a device used in many different types of technologies, including transporters, photon torpedos, and sensor arrays.
In 2151, when Malcolm Reed attempted to beam Crewman Ethan Novakovich up from the surface of a planet during a powerful windstorm, Novakovich rematerialized with debris embedded in his skin, due to the transporter's phase discriminator's inability to isolate the debris from the matter stream. (ENT: "Strange New World")
In 2152, Reed was able to use a phase discriminator to decloak a device placed on the hull of one of the Enterprise NX-01's shuttlepods designed to generate a plasma stream. (ENT: "Shockwave")
In 2368, most of the command crew of the USS Enterprise-D were able to use a phase discriminator, in conjunction with a subspace force field, to to phase-shift into the time continuum of the Devidians, aliens who were using a synchronic distortion with a positive displacement of .004 percent to conceal themselves. Later, they outfitted photon torpedoes with phase discriminators to re-phase the force of the explosions into the time continuum of the Devidians. (TNG: "Time's Arrow", "Time's Arrow, Part II")
The next year, Captain Picard, Lieutenant Commander Data, Lieutenant Commander La Forge, and Counselor Troi utilized the same procedure, using the type-seven phase discriminators contained in emergency transporter armbands, to protect themselves from becoming integrated into the timeframe of a temporal fragment in which the Enterprise had become trapped. (TNG: "Timescape")
In 2370, as the Enterprise crew was testing a new photon torpedo guidance system designed by Lieutenant Worf, a radial imbalance in the phase discriminator of one of the torpedo's primary guidance module caused it to go off course. (TNG: "Genesis")
A biospectral phase discriminator was a standard component of a runabout's sensor array. In 2372, Doctor Julian Bashir ordered Chief Miles O'Brien to remove the discriminator for the Rubicon, as he required the device to further his investigation into developing a means whereby he could cure a group of rogue Jem'Hadar from their addiction to ketracel-white. O'Brien, however, used the opportunity to utilize the runabout's transporter to escape from the Jem'Hadar who was guarding him. (DS9: "Hippocratic Oath")
Later that year, after the USS Voyager had been duplicated by a subspace divergence field, the crew rigged a portable phase discriminator which could protect anyone passing through a spatial rift which linked the two ships from the deleterious effects of the spatial transition. (VOY: "Deadlock")