Definition:
When you have too many tools at your disposal. You may use some tools for a particular purpose only. Eventhough that tool is capable of more, you will not bother as the choices are many and particularly there is not much time.

Then one day as you look out for a latest tool to make a particular type of work, you suddenly remember that a little used feature of this tool and another feature of another tool will make ideal partners. Those old tools suddenly finds new usage and becomes the rage all over.

Thus we use Javascript for mouse roll over effects & validating HTML form entries for ages. It is a full fledged script, but we don't care much. Eventhough javascript can be disabled at user's computer, we take it for granted and now it almost becomes a standard.

The strength of Javascript is ofcourse, displaying messages, pictures etc without refreshing the page. In short, we can change the page content & looks without waiting for the page to re-load from web server.

And then, there is this feature in XML where you can ask the server with a specific data and receive just that particular data. You don't have to receive the entire page as in normal HTML form submissions.

Now, when a particular button is clicked or an entry is made in a web page, using XML you can send just a particular query & send get short answer from webserver, the page is not re-loaded (or refreshed) at this point. You then use Javascript to change the page looks using the short answer from the server.

The result? Changes appear instantaneous. As a bonus,the web server sends only the relevant part, so it's work load is also reduced. Now, such a unique technique of combining two long existing tool must have an acronym!!

This technique was named as 'Asynchronous Javascript and XML'. And abbreviated in short as 'AJAX'! What a stylish name to go with the magic effect!! We geeks got one more acronym to gleefully confuse & intimidate lay users.

A new technique, not a new technology:
But honestly, this is one technique that can change the way the websites are accessed. That's right, AJAX is not a new technology, but a new technique of combining two old technologies.

Who invented the term?
The term was first used by Jesse James Garett in his article "Ajax: A New Approach to Web Applications" at AdaptivePath on February 18, 2005.

What are the downsides?
The back button in your browser may not take you to previous state of the page as expected.