distress signal

noun

1. a signal used, or designed to be used, by persons in peril, for the purpose of summoning aid, indicating their position, etc., as a radio code sign, aerial flare, flag hoist, or the like.Compare distress call(def 1).
2. an indication, especially a nonverbal one, that assistance, cooperation, or the like, is needed: He correctly interpreted the host’s upturned eyes as a distress signal and hastily changed the subject.

noun

1. a signal by radio, Very light, etc from a ship or other vessel in need of immediate assistance