What does the ‘x’ in the extensions aspx, docx, xlsx, etc. represent?

Accepted answer +11 upvotes

For .aspx I assumed it stands for:

Active Server Page eXtended format

Though another opinion is that:

these files typically contain static (X)HTML markup, as well as markup defining server-side Web Controls and User Controls

Apparently it was the cool thing to do at time (the quote actually talks about the original name XSP, but doesn’t rule it out as an option):

The initial prototype was called “XSP”; Guthrie explained in a 2007 interview that, “People would always ask what the X stood for. At the time it really didn’t stand for anything. XML started with that; XSLT started with that. Everything cool seemed to start with an X, so that’s what we originally named it.”

For the office documents, since they are in XML format, it stands for XML.