Ajax is a brand-new suite of programming solutions.
[ Seth's Blog: What to do if you don't know what to do ]
In addition to getting called on the real reason Chicago is called the Windy City, he's a bit off about what AJAX really is.
Wikipedia is quick to point out that "Ajax is not a technology in itself, but a term that refers to the use of a group of technologies together." and it is simply a cutsie acronym for "Asynchronous JavaScript And XML".
Modern AJAX techniques tend to rely on the javascript XMLHttpRequest but earlier techniques (like the kind I used) invoved an IE-only concept called XmlDataIslands (props to Nate for bringing that one to my attention). The term Ajax (which should be AJAX) was coined by Adaptive Path and has quickly gained favor, especially among the "Web 2.0" crowd. However it must be pointed out that this asynchronous dynamic has been used for years, and it nothing new at all.
And the rest of Ajax, notably the Javascript and XML are even older...
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Disclaimer The opinions expressed herein are my own personal opinions.