A proximity alarm was an alert used in Starfleet vessels, to warn the pilot that another shuttle was getting too close or that their own shuttle was coming dangerously close to another vessel.
In 2151, a proximity alarm sounded on a shuttlepod of the Enterprise as it approached within 40 meters of a Klingon Bird-of-Prey. (ENT: "Sleeping Dogs")
In 2368, Cadet Wesley Crusher noted in his deposition at the inquiry into fellow Cadet Joshua Albert's death, that his proximity alarm went off near Saturn's moon Titan when Albert's shuttle flew too close to his. In reality, Nova Squadron, which Albert and Crusher were members of, attempted to perform the Kolvoord Starburst, a maneuver banned from Starfleet Academy since the 23rd century, during which there was an accident and Albert was killed. (TNG: "The First Duty")