MEAN is an acronym that represents a modern web development stack used for building web applications. Each letter in "MEAN" stands for a specific technology or component that is commonly used in MEAN stack development:

1. MongoDB: MongoDB is a NoSQL database that stores data in a flexible, JSON-like format called BSON (Binary JSON). It is designed to handle large volumes of unstructured or semi-structured data and is particularly well-suited for applications where data schemas can evolve over time. MongoDB is used as the database component in the MEAN stack.

2. Express.js: Express.js is a web application framework for Node.js, a JavaScript runtime environment. Express.js simplifies the process of building web applications and APIs by providing a set of tools and features for handling routing, middleware, and HTTP requests. It serves as the backend framework in the MEAN stack.

3. Angular (or AngularJS): Angular is a front-end web application framework developed and maintained by Google. It allows developers to build dynamic, single-page web applications with a structured and component-based architecture. Note that there are two major versions of Angular: AngularJS (1.x) and Angular (2+). Depending on the project, developers may use either version.

4. Node.js: Node.js is a server-side JavaScript runtime that allows developers to run JavaScript code on the server. It is designed for building scalable and high-performance network applications. In the MEAN stack, Node.js is used to run server-side code and interact with the database (often MongoDB) through Express.js.

The MEAN stack is known for its full-stack JavaScript approach, which means that developers can use JavaScript for both the front-end and back-end of web applications. This consistency in the programming language can simplify development and foster code reuse. MEAN is particularly popular for building real-time applications and single-page applications (SPAs).