A means asynchronous, which is an attempt to say that an XMLHttpRequest object supports client-server data exchange without the need to refresh the Web page. On the other hand, it does not support server push but uses polling instead. So calling this asyncronous communication is a stretch to say the least.
X stands for XML, but XML is not actually needed for using XMLHttpRequest object. I’d say it’s not even recomended as it increases the amount of data that has to go through the wire and hurts the responsivenes of AJAX applications.
Here comes the puzzle (ask your kids for an answer):
A, Ja, and X were sitting on an axe
A has lost her head,
And X is also dead
Who’s left sitting on the axe?