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.

Origin:
1870–75

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World English Dictionary
distress signal
 
n
a signal by radio, Very light, etc from a ship or other vessel in need of immediate assistance

Collins English Dictionary - Complete & Unabridged 10th Edition
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Encyclopedia Britannica
Encyclopedia

distress signal

a method by which a ship at sea can summon assistance. Distress signals are fixed by custom and by internationally agreed-on rules of the road at sea. The most important are: (1) visual signals, such as a flame, a red flare, an orange smoke signal, or a square flag displayed with a ball below; (2) sound signals, such as a gun or rocket fired at regular intervals, or a continuous sounding of a fog-signal apparatus; and (3) radio signals such as the Morse group SOS, the international code signal NC, or the spoken word "Mayday" (from French m'aider, "help me"), by radiotelephone. Distressed vessels may also actuate alarms of other vessels by a radio signal consisting of a series of 12 four-second dashes or by a radiotelephone signal consisting of two tones alternately transmitted for 30 to 60 seconds

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
We were rudely shaken from these rosy dreams by a distress signal from the bike.
The universal distress signal for choking is grabbing the throat with the hand.
The direct cause is a chemical distress signal produced in skin that is damaged
  by another hazard of modern life: eczema.
Computer simulations suggest the distress signal should still be capable of
  being detected.
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