Node.js is an open-source, cross-platform JavaScript run-time environment that executes JavaScript code on server-side.

JavaScript was primarily designed, embedded and used in HTML for providing interactivity in web pages. It gets executed by JavaScript engine present in the user's web browser. But, Node.js extended the use of same old/familiar JavaScript for server-side scripting i.e., for running scripts on server-side to produce dynamic web page content before the page is sent to the user's web browser.

Node.js unified web application development around a single programming language, rather than different languages for server side and client side scripts.