Ajax is not a programming language or a tool, but a concept. Ajax is a client-side script that communicates to and from a server/database without the need for a postback or a complete page refresh.

The best definition I’ve read for Ajax is “the method of exchanging data with a server and updating parts of a web page – without reloading the entire page.”

Ajax itself is mostly a generic term for various JavaScript techniques used to connect to a web server dynamically without necessarily loading multiple pages.

In a more narrowly-defined sense, it refers to the use of XML Http Request objects to interact with a web server dynamically via JavaScript.