AJAX (Asynchronous JavaScript and XML) is a set of web development techniques that use various web technologies on the client side to create asynchronous web applications. AJAX allows web pages to be updated asynchronously by exchanging small amounts of data with the server behind the scenes. This means that parts of a web page can be updated without reloading the entire page, resulting in a more dynamic and responsive user experience. AJAX involves using a combination of HTML or XHTML, CSS, JavaScript, and the XMLHttpRequest object. It is commonly used for real-time data updates, form submission, and interactive user interfaces.