Twig is a template engine for the PHP programming language. It is used to build dynamic HTML, XML, or other markup languages by embedding PHP code into templates. Twig is known for its simplicity, flexibility, and security, making it a popular choice for developers building web applications. Twig is widely used in the Symfony PHP framework, but can also be used as a standalone template engine in other PHP applications. It is available as open-source software and is released under a BSD license.