Emergency signal

From Memory Alpha, the free Star Trek reference

The emergency signal is a communicator function that indicates the holder is in distress and requires immediate help.

In 1969, the USS Enterprise received an emergency signal during Kirk and Sulu's mission to retreive evidence. Believing the landing party was in jeopardy, Spock ordered an immediate beam-up, and brought aboard an Air Police Sergeant who had confiscated Kirk's communicator and inadvertently activated its emergency signal. (TOS: "Tomorrow is Yesterday")