Selenium is a free (open-source) automated testing framework used to validate web applications across different browsers and platforms. You can use multiple programming languages like Java, C#, Python etc to create Selenium Test Scripts. Testing done using the Selenium tool is usually referred to as Selenium Testing. 

Selenium Software is not just a single tool but a suite of software, each piece catering to different testing needs of an organization. Used by Micro Focus, BrowserStack, SauceLabs

Relesed in 2004, developed by Jason Huggins