A perimeter alert was an alert used by Starfleet and the Cardassian military, to warn about an approaching, unidentified starship.

In 2364, Wesley Crusher, while sitting in the command chair on the USS Enterprise-D, flipped open a control panel and told Captain Jean-Luc Picard of a perimeter alert when an alien entity approached Deneb IV. When Picard ignored him, Lieutenant Worf informed him of the same alert. (TNG: "Encounter at Farpoint")

In 2371, a Cardassian perimeter alert in the Almatha sector was believed to be caused by the Maquis-controlled USS Defiant, which later turned out to be false. Outpost 47 then reported another perimeter alert, this time actually caused by the Defiant. (DS9: "Defiant")

In 2372, Tom Paris announced a perimeter alert when the Vidiians approached the USS Voyager. (VOY: "Deadlock")

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