Open-source refers to a type of software whose source code is available to the public, free of charge. This allows anyone to view, modify, and distribute the software for any purpose. Open-source software is typically developed and maintained by a community of volunteers, and it can be a cost-effective alternative to proprietary software. It is used in a wide range of applications, from operating systems and databases to mobile apps and games. Open-source software is also known for its transparency and security, as the community can review and improve the code.