Signal is a cross-platform encrypted messaging service developed by the Signal Foundation and Signal Messenger LLC. It uses the Internet to send one-to-one and group messages, which can include files, voice notes, images and videos. Its mobile apps can also make one-to-one voice and video calls, and the Android version can optionally function as an SMS app.

Signal uses standard cellular telephone numbers as identifiers and uses end-to-end encryption to secure all communications to other Signal users. The apps include mechanisms by which users can independently verify the identity of their contacts and the integrity of the data channel.

All Signal software is free and open-source. The clients are published under the GPLv3 license, while the server code is published under the AGPLv3 license.