Condition Red was an alert status or code word used by various administrations and command structures to bring personnel to an alert status.
On Earth in 2008 it was used by the Saxon Master's government to imply a plan had gone wrong. Miss Dexter referred to Clive Jones warning Martha Jones to stay away from the family home as a Condition Red. (TV: The Sound of Drums)
When Martha's family and Jack Harkness tried to assassinate the Master, a Condition Red alert went out. (TV: Last of the Time Lords)
In 2012, during the Van Statten Incident, Bywater announced a Condition Red in the Vault when the Dalek known as "the Metaltron" became active and killed Danny Simmons. (TV: Dalek)
Utilised by the Earth Empire Security Commander Condition Red allowed them to shoot dead anyone he or she felt presented a situation. John Jefferson threatened to shoot Toby Zed on Sanctuary Base 6 under the terms of such an alert. (TV: The Impossible Planet, The Satan Pit)