AJAX isn't something that is easy to define today. Looking at what it stands for - Asynchronous JavaScript and XML - the term stands for nothing more than a technique that involves making asynchronous (non-blocking) requests using JavaScript that retrieve XML data. Nothing else.

However, today, the term is becoming more and more vague in meaning as it's used in promotion of products. Thanks to its vague definition, many people see it as a good and complex thing that makes a product awesome.

I won't try to convince you that it should only be used to define something that uses JavaScript to retrieve XML data, so I'll concede that, today, it means most of the points you (codeexpert123) wrote in your post. Mentioning XHTML and CSS together with it is pushing it though, but that association probably comes from Microsoft's contribution to AJAX-enabling a website. Some of their "AJAX" controls don't even have the slightest to do with AJAX (most of the control extenders don't.)