Node.js is an open-source, cross-platform JavaScript runtime environment that allows developers to run JavaScript code outside a web browser. Built on Google’s V8 JavaScript engine, Node.js is designed for high performance and uses an event-driven, non-blocking architecture to handle multiple requests efficiently. It is widely used for building fast and scalable server-side applications, RESTful APIs, real-time applications, and microservices. Node.js also includes a large ecosystem of open-source packages through npm (Node Package Manager), enabling developers to quickly add functionality to their projects. Its speed, scalability, and ability to use JavaScript for both front-end and back-end development make Node.js a popular choice for modern web application development.