What is the MX branding for??

So far we have seen that Microsoft is making some of their legacy software into "MX" applications for Windows 8 and Phone with the OneNote MX app. It was really good and you can read my review here. My question is what really is the true meaning of MX? Is it Microsoft Xtreme? I have heard some speculation of a MovieMakerMX and a PhotogalleryMX. What are your opinions?