Open source refers to a software development and distribution model that makes the source code of a program publicly available to anyone who wishes to view, modify, or distribute it. This means that the source code, which is the underlying instructions that make the software work, can be freely accessed, modified, and shared by anyone.

Open source software is often developed collaboratively by a community of programmers who contribute to its development and improvement. This collaborative approach can lead to faster innovation, better quality software, and lower costs.

Examples of open source software include the Linux operating system, the Apache web server, the WordPress content management system, the Mozilla Firefox web browser, and the LibreOffice productivity suite. Open source has also been applied to other fields, such as open-source hardware, open-source robotics, and open-source medicine.