What does X denotes in ASPX, DOCX, XLSX, PPTX etc?
It's one of those things you just take for granted until one day someone asks you and you realize you can't answer it. Much like for years I never questioned the use of 1033 directories in Microsoft products for years until one day, someone asked me about it.
Around the release of .NET and Office 2007, Microsoft added an x to basically all of their extensions and I frankly took it as representing XML, but that simply doesn't make sense with .aspx.
So, I realize this is a very non technical question, but now that the question has been asked of me and my googling hasn't given me an answer, can anyone tell me with authority what the X represents? Is it extended? Xml? Or is there no meaning behind it?