Yellow alert
From Memory Alpha, the free Star Trek reference.
A yellow alert (also condition yellow or code yellow) is the second highest alert signal status on Starfleet starships, one stage below red alert. It designates a ship-wide state of increased preparedness for possible crisis situations. In this status, shields and deflectors are brought to full power, however the weapons systems remain off line.
A yellow alert may either be activated manually by the commanding officer in preparation for a tactical or environmentally hazardous situation, or automatically upon detection of unknown or hostile spacecraft, or upon detection of certain system malfunctions. If the situation escalates, the commanding officer or the main computer will order a red alert.
Captain Benjamin Sisko put Deep Space 9 on yellow alert after learning that the Dominion was about to invade the Alpha Quadrant. (DS9: "In Purgatory's Shadow")
- It appears that bringing shields to full power during yellow alert was not standard practice until some time in the 2280s, as only defense fields were raised during yellow alert before then. (Star Trek II: The Wrath of Khan)
Admiral Haden informed Captain Jean-Luc Picard that all Federation starships were on yellow alert in preparation for a possible Romulan invasion from Nelvana III. (TNG: "The Defector")
- This implies that a yellow alert can be a general fleet-wide order from Starfleet Command or the Federation Council.
