AJAX (Asynchronous JavaScript and XML) is a web development technique that enables web pages to update specific content without reloading the entire page. By sending asynchronous requests to the server, AJAX can retrieve or send data in the background. This enhances user experience by making web applications more interactive, faster, and dynamic, as only the necessary data is fetched or updated. Commonly used with JSON, AJAX powers features like live search, form validation, and dynamic content loading in modern web applications.