AJAX (Asynchronous JavaScript and XML) is a technology that enables websites and applications to retrieve and update data asynchronously (i.e. without having to reload the page) from a server. AJAX combines JavaScript, XML (or JSON) and HTTP requests to enable dynamic and interactive websites and applications.

In contrast to traditional websites, where the entire page is reloaded each time the user takes action, AJAX allows parts of a page or dynamically generated content to be updated quickly and smoothly without having to reload the entire page. This increases the speed and user-friendliness of websites and applications and improves the overall user experience.

A typical example of the use of AJAX is a live chat feature on a website. When a user sends a message in the chat window, the message is transmitted asynchronously to the server and the server's response is then updated asynchronously on the page without the page having to be reloaded. This makes communication faster and more fluid.

AJAX is supported by many popular web frameworks such as jQuery, AngularJS, React and Vue.js and is an important technology for the development of modern, dynamic and interactive websites and applications.