A signal may refer to any of the following:
1. In electronics, a signal is an electrical pulse that is used as a method of transmitting data.
2. Signal is a secure messaging application developed by Open Whisper Systems for iOS and Android, with desktop versions available for Linux, Windows, and macOS. It encrypts data transfers such as text messages, images, video, and voice calls, with one or multiple recipients. It uses end-to-end encryption, enabling users to communicate with maximum privacy. A mobile phone number is required to register for a Signal account, even for the desktop application.
The Signal server and client are open-source, which allows anyone to scrutinize their code for weaknesses or vulnerabilities. More information and downloads are available at the Signal homepage, https://signal.org/, and the Signal source code is available at https://github.com/signalapp/.
3. Signals are software interrupts that are used for interprocess communication in UNIX and UNIX-like operating systems such as Linux. See our Linux signals guide for further information on signals.