Mayday

Supervisor Lowe sends out a Mayday after the Swarm attacks Titan Base. (TV: The Invisible Enemy)

"Mayday" was a codeword, used to signal distress.

Munro sent out a Mayday call when the Prince Charlie oil rig was attacked by a Skarasen. (TV: Terror of the Zygons)

Discovering that Titan base was under attack from persons infected by the Swarm, Supervisor Lowe sent out a Mayday distress call. (TV: The Invisible Enemy)