Communications
- Beacon
- Distress signal
- International maritime signal flags, using the International Code of Signals
- Maritime flag signalling, generally flaghoist signalling, by which ships communicate with each other
- Naval flag signalling, covers various forms of flag signalling, such as semaphore or flaghoist
- Recognition signal in human, technical or biological communications
- Unix signal, a form of interprocess communication
- Signal (electrical engineering), a function that conveys information about the behavior or attributes of some phenomenon
- Signals (military), military communications
- Signalling (telecommunications), a part of some communication protocols
- Smoke signal
- Telegraphic signal, in telegraphy
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