MEAN stack stands for MongoDB-Express-AngularJS-NodeJS.

First of all, MEAN is JS-only environment. Express is one of the most popular web app frameworks for NodeJS. Node.JS development often goes along noSQL database, like CouchDB or MongoDB, due to their document based structure and JSON notation, which is suitable for all-JS nature of NodeJS. AngularJS is a lightweight framework for building client-side applications.