One stage above a yellow alert, this status had weapons systems and shields brought to full power, essentially preparing the ship for combat. The Klingon equivalent of this status was defense condition one. (Star Trek III: The Search for Spock; DS9: "Once More Unto the Breach")
Red alert could also be ordered during various other emergency situations, such as a ship being boarded, radiation exposure, a security alert, massive systems failure, an imminent warp core breach, and an evacuation order among others. During a red alert, lights were dimmed, alarms sound, and computer terminals showed constant red graphics.
A red alert could either be activated manually by the commanding officer or the officer in charge, or automatically, as when a ship was attacked, or entered a dangerous area, etc. (TOS: "The Corbomite Maneuver") The raising of shields automatically triggered a red alert. (TNG: "Contagion")
Sections of a starship could be placed on red alert while other sections remained at a lower alert status. (Star Trek VI: The Undiscovered Country)
In 2293, while the USS Enterprise-A was returning from Rura Penthe, an intercom message informed the crew that only certain decks should remain on red alert while in Klingon space. (Star Trek VI: The Undiscovered Country)
In 2369, Commander Benjamin Sisko ordered red alert for Deep Space 9 when an alien ship from the Gamma Quadrant fired a radiation burst onto the shields, which depolarized them. (DS9: "Captive Pursuit")
In 2372, Captain Sisko ordered red alert on the USS Defiant, shortly before the cloaking device was deactivated and the shields raised, to prepare for the battle with three Klingon Birds-of-Prey. (DS9: "The Way of the Warrior")
|Colored alerts:||Red • Double Red • Yellow • Blue • Gray|
|General alerts:||Battle stations • "Brace for impact" • General quarters|
|Battle • Evacuation • Fire • Intruder • Perimeter • Proximity • Security • Standby • Tactical|
"Red alert" comes from the naval tradition of general quarters, and in fact the command "general quarters" may be used interchangeably to bring the ship into red alert.
In "The Corbomite Maneuver", red alert was referred to as Condition and General Alerts.
In "Scorpion, Part II", Chakotay mentioned that the USS Voyager would remain on "full" red alert. This may signify that there are multiple levels of red alert status, or that certain aspects of red alert, such as manning battle stations, can be stood down while allowing other aspects such as charged weapons to remain active. It may however simply refer to the fact that Voyager would remain at red alert for the duration.
In Star Trek: The Next Generation, the red alert lights change during the show's run. The first time red alert is heard is in "Encounter at Farpoint", but the lights aren't flashing red. The first time the lights are seen flashing red is in "The Naked Now", which have only the top and bottom part of the lights flashing. However, beginning in "Where Silence Has Lease", the whole part of the lights flash. The next change the lights have is seen in "Man of the People", where there is a longer period between when it flashes and when the sound is heard.