An alarm was a device that gives an audible, visual or other forms of signal to inform people about a problem or condition.
Alarms sounded on Platform One when metal spiders belonging to the Lady Cassandra sabotaged the station's forcefields, leaving everyone onboard unprotected from the deadly heat of the expanding Sun. (TV: The End of the World)
After using Rose Tyler's body to overpower the Tenth Doctor inside the New New York Hospital's intensive care unit on New Earth, the Lady Cassandra ripped a bunch of wiring from the wall and set off an alarm in order to summon the Sisters of Plenitude. (TV: New Earth)