AJAX (an acronym for Asynchronous JavaScript and XML) is a group of interconnected web development techniques running on a user's computer. AJAX is primarily used to create asynchronous web applications.

Thanks to AJAX, web applications can receive and send data asynchronously (i.e., in the background) without the ongoing communication having to interfere with the appearance or behavior of the page itself. However, AJAX is not a single technology, but a whole group. AJAX consists of HTML, CSS, DOM, JavaScript and XML/JSON.

One of the first widespread deployments of AJAX is known to be Google, which used the technology for its then-fledgling Gmail email (2004) and for Google Maps (2005).

Some of the drawbacks of AJAX:

• In some browsers, an updated AJAX page may not be logged in the history, so the „Back“ function then returns the user not to the previous version of the AJAX page, but to the previous „full“ version of the site.
• In some web applications, dynamic background AJAX updates may interrupt user actions, especially if the user is working on an unstable Internet connection.
• Some user interfaces built on AJAX can significantly increase the number of requests to the server and to databases, which can lead to longer time lags between server responses.
• User interfaces that use AJAX extensively require a lot of performance from the browser, which must process large and complex scripts and often re-render complex sites. This results in slow and „choppy“ websites that are not responsive to the user and also put more load on the computer's processor.