A JavaScript Runtime is an environment that allows JavaScript code to be executed. It provides the necessary tools and APIs for running JavaScript outside of a web browser. The most common JavaScript runtime is Node.js, which enables developers to run JavaScript on the server side, making it possible to build scalable network applications. In addition to Node.js, other JavaScript runtimes include Deno and the JavaScript engines found in web browsers, such as V8 in Google Chrome. These runtimes handle tasks like memory management, event handling, and executing code, ensuring that JavaScript can perform efficiently in various environments.