Not to be confused with the cleaner AJAX.

Ajax (also AJAX; /ˈeɪdʒæks/; short for “Asynchronous JavaScript And XML”)[1] is a set of Web development techniques using many Web technologies on the client side to create asynchronous Web applications. With Ajax, Web applications can send and retrieve data from a server asynchronously (in the background) without interfering with the display and behavior of the existing page. By decoupling the data interchange layer from the presentation layer, Ajax allows for Web pages, and by extension Web applications, to change content dynamically without the need to reload the entire page.[2] In practice, modern implementations commonly substitute JSON for XML due to the advantages of JSON being native to JavaScript.[3]

Ajax is not a single technology, but rather a group of technologies. HTML and CSS can be used in combination to mark up and style information. The webpage can then be modified by JavaScript to dynamically display – and allow the user to interact with — the new information. The built-in XMLHttpRequest object within JavaScript is commonly used to execute Ajax on webpages allowing for websites to load content onto the screen without refreshing the page. Ajax is also not a new technology, or another different language, just existing technologies used in new ways.

Source: https://en.wikipedia.org/wiki/Ajax_(programming)