A metaphasic sweep is a type of scan that can be used to detect a cloaked vessel.

When an old Klingon D7 class battle cruiser attacked the USS Voyager, believing that the Empire was still at war with the Federation, Seven of Nine used a metaphasic sweep to reveal its location so that Voyager could disable its cloak. This worked due to the relatively antiquated technology of the cloaking device. (VOY: "Prophecy")

Cloaking penetration technology
Antiproton beam • Gravitic sensor net • Long-range tachyon scanner • Metaphasic sweep • Quantum beacon • Tachyon detection grid