Perimeter alert
From Memory Alpha, the free Star Trek reference
A perimeter alert was an alert used on Starfleet vessels, to warn the crew of 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. (TNG: "Encounter at Farpoint")
