The highest alert condition aboard a Starfleet vessel, calling the crew to battle stations with sirens and flashing red lights when the ship faces immediate danger.
Red alert is the top level of readiness on Starfleet ships and stations. When it is called, a klaxon sounds, red warning lights flash, the crew go to their stations, and the ship's defences are brought to full readiness. It may be ordered by the officer in command or set off by the computer.
The signal is heard from the original series onwards and in every later series and film, and is as much a part of starship life as the captain's log. The 22nd-century Enterprise has no such system at first. In Singularity, Malcolm Reed works out a ship-wide tactical alert that is its forerunner.
Lesser conditions include yellow alert, and some ships add others of their own, such as the blue alert used when Voyager lands and the black alert that warns Discovery's crew of a spore drive jump.