Metaphasic sweep

A metaphasic sweep was a type of scan that could 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.