Colonel William A. Phillips

A signal is any variation of a medium that conveys information.

Signal or signaling may refer to:

Science and technology

SIGNAL (programming language), a synchronous programming language
Signal (messaging app), an end-to-end encrypted voice calling, video calling and instant messaging application
- Signal Protocol, a cryptographic protocol used by the Signal application
Signal (IPC), a form of inter-process communication used in Unix and related operating systems
- C signal handling, a way for handling signals received by programs during their execution
A term in functional reactive programming

Military communications, often called "signals"
Recognition signal in human, technical or biological communications
Signaling protocol to establish or tear down a connection, and to identify the state of the connection
Signaling (telecommunications), a part of some communication protocols
Smoke signal, an ancient form of communication
Telegraphic signal, in telegraphy

International Code of Signals, a standard for sending signals by displaying flags
- 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
Railway signal
Traffic light

Signal (magazine), a Wehrmacht propaganda magazine which was published in occupied Europe during World War II
SIGNAL (US magazine), publication of the Armed Forces Communications and Electronics Association
Signal, a New York Yiddish-language literary monthly published from 1933 to 1936 by the Communist Party USA
Signal (novel), a 2009 children's novel about a boy who helps a girl from another planet signal her parents
Signal (South Korean TV series), a 2016 South Korean police procedural TV series
Signal (Japanese TV series), a 2018 Japanese police procedural TV series based on the 2016 South Korean series of the same name
Signal – The Southeast Electronic Music Festival, an annual music festival held in the US
Signal, Wyoming, a fictional town from the 2005 film Brokeback Mountain
"Signals", a 2005 episode of the animated television series 12 oz. Mouse

Signal (bridge), a permitted means of conveying information to partner through the play of cards in contract bridge
Citation signal, in law
Signal (toothpaste)
USS Signal, several US Navy ships
Signal corps, military branch responsible for military communications