Selenium is an open-source automation tool used for testing web applications across different browsers and platforms. It enables testers to write scripts in various programming languages, such as Java, Python, and C#, to simulate user interactions and validate application behavior. Selenium supports automated testing, reducing manual effort and enhancing testing efficiency for web applications.

Selenium includes a suite of tools like Selenium WebDriver, Selenium IDE, and Selenium Grid, which allow for flexible testing across multiple browsers and distributed environments. It’s popular for its ability to handle complex web applications and support continuous integration (CI) practices, making it a top choice for quality assurance in agile development.