A distress beacon (also known as a disaster beacon) was an automated device on board starships or shuttles that was used to alert other ships or stations that they were in need of help, and made it easier to locate the starship in distress. It was typically deployed in dire circumstances, such as immediately before a starship is disabled or destroyed.
In 2267, the signal from the automated disaster beacon of the USS Constellation was barely detectable by the USS Enterprise as the Enterprise attempted to find the Constellation. (TOS: "The Doomsday Machine")
The distress beacon of the Promellian battle cruiser Cleponji continued to transmit for at least a millennium after its crew had perished while their ship been held by aceton assimilators, until it was silenced at long last after being discovered by the USS Enterprise-D in 2366. (TNG: "Booby Trap")
After crash landing on a Class D planet in 2375, Tuvok re-calibrated his shuttle's distress beacon. The beacon was later used as the center of a two-meter radius where a transporter beam from the USS Voyager transported Tuvok, Tom Paris and Noss back to the ship. (VOY: "Gravity")